My home realm has many knights, several of which have held their title longer than Belegarth has been its own sport. That isn't exactly common, but many realms have such experience, with or without the title. Knights here are allowed one squire, and a second if they call and pass a vote to allow it. With so many knights, that means there can be many people going through the squiring process at any given time. They only true requirement for them to get knighted is to earn enough votes from the council of knights.
I say that's the only true requirement, but the rest of the requirements come from their knight, and the standards the other knights will use to decide how to vote. The realm's general requirement is that each squire must beat all of the active knights in challenges, set by the knights. These can be duels, field challenges, or non-fighting displays of skill or knowledge. Many knights gauge their squire against Kyrian's list of squire tasks, as it encompasses nearly every type of task one could give a squire. Once a knight feels their squire has completed their tasks, they will bring them up for a vote to be knighted.
The knight and squire relationship is important to the process. In addition to their direct training, knights apply their experience to guiding their squire through the process. Occasionally, this can be harsh truths, comforting words, or a kick in the right direction. They help keep the squire on task and motivated to finish challenges and tasks. They also need to point out weaknesses that need improved and extra tasks (like running a specific event). The main thing the squire needs is a little bit of help outlining what they need to work on and where they stand. Some of this could be done by a realm taking on a squire in lieu of a knight. Those realm squires, without a knight, do miss out on a little bit of the fun part of squiring--fighting alongside their knight.
One misconception that seems to be common is that a squire learns everything from their knight. While a knight teaches their squire many things, a great deal of learning comes from others. Squires must be on a quest to learn from as many people as possible, while doing their best to think critically of their own techniques. A lot of skill develops from self-evaluation and adapting to overcome skilled opponents. By challenging the best fighters and seeking their criticism, one can often find what needs to improve, but it takes concentrated effort to actually fix technique. This also applies to service, as most knights have much experience in planning and running events, and can be great at recruitment and retention. Even in areas that have few knights (or those of equivalent experience), a realm might be able to pass along its shared experience and knowledge to a squire who seeks to learn all their area has to offer. However, one may need to venture beyond their realm to gain more specific knowledge that elder realms/knights might be able to offer.
As a squire, I always set for myself the goal to "do what a knight should do". That means a lot of different things to each person, but gives one a guideline to follow. If there is a gap in knowledge or skill that prevents achieving that goal, then obviously that is something that needs to be worked on and improved. It also frames the mindset of service, as in if I would complain that a knight should be doing a task, why wouldn't I just take care of it? Of course, this also requires an idea of what knights do, or to have knights to emulate.
The answer to the problem is complicated, as each knight brings something different to the table. There are some that make a distinction between "fighting knights" and "service knight". All knights should be both, to varying degrees. Fighting skill (or knowledge of fighting technique) is important for building a stronger realm and can be a great way to bolster recruitment through displays of top skill. Service is a broad category, but even knights that focus on fighting still serve the realm through teaching. It is also helpful for event planners to have fighting experience so they can plan fighting that keeps attendees involved and entertained. The point here is that both sides bolster each other, and it is important to develop in both areas.
One aspect of fighting skill that is often overlooked in discussion of squire tasks is field leadership. Taking command and helping the group find success on a large field is something knights should be doing (see above). Many knights have their squires read classic books on military strategy (Art of War, etc) to help build this sort of strategic mindset. Often, squires will be tasked to lead realm battles or shepherd a flock of new fighters to test their leadership. Developing a keen eye for the flow of a line fight and gaps in the enemy line not only boosts their ability to personally influence the fight, but also to lead others to the same ends. Smaller groups/realms might find that having a reliable commander might make the experience at major events more fun as they find more success on the field.
Knights don't have to be the best one on one fighters in every style. However, they do need to be well versed in a variety of weapons and techniques. Most knights end up specializing in a few weapons or styles, and are often experts in one or two. I would say "mastered" one or two styles, but I have always felt that there will always be room to improve. At the end of a squire's path to knighthood, they should be able to display a high level of skill in a couple of styles. Even being an expert in a support weapon should qualify. Controversial as it may be, even a non-combatant might be able to display fighting knowledge that rivals even veteran fighters--leaving the path of non-com squire open to those realms that would find it fitting. To fit the mold of "someone that could run a realm", a knight needs to be able to teach fighting technique, not necessarily excel at it. For example, I'm no expert on flails or bows, but I could teach someone how to use them effectively.
All squires should be experts on the Book of War (and/or Dag's Manual of Arms). And by expert, I do mean expert. They will need to know the rules for combat to be a herald, and the rules for weapon construction to check weapons and to build them. Even someone that has been in the sport for a while should review the rules and study them a little bit periodically, especially if one plans to work towards knighthood. The hard part for newer realms, or those farther from the traditions, is that it is difficult to learn unwritten rules and old combat conventions without exposure to long serving veteran fighters. This is where disparity appears between realms and regions, especially in realms that rarely make it to large events. Knights pass along this tradition to their realms. Squires in more isolated realms should make it a point to visit older realms and learn from elder knights.
Squires/knights should do their best to be paragon of sportsmanship and a good representative of the sport. They need to be an example to others of how great fighting can be. By having fun, fighting hard, and coming away from a fight with a positive attitude, they can help make a mediocre day of fighting into a great time for everyone on the field. From a recruiting standpoint, they need to be able to talk to newcomers, teach them the basics, and get them fighting. They should be approachable and helpful. Consider who recruited you to the sport, who really sold you on sticking around, and do your best to emulate them.
The benchmark for when a squire is ready to be knighted should be high. The process should be hard enough to forge a strong fighter and realm leader. It should last long enough to give a squire opportunity for growth and success, as well as a chance to fail and recover, but each one's path is different and takes its own time to complete. There should be no rush to be knighted, for the more difficult and time consuming the challenge, the more rewarding the title will be.
A bit of a side note. Several people concern themselves with the prestige or reputation of the title "knight". While it is something that many people honor, the meaning of the title rests in the hands of those who have earned it. Anyone can call themselves a knight, but those that have earned the respect of the knights they have aspired to be will find their title much more meaningful. The title is more that you have been accepted to join them as a peer of the order. For those knighted under realms, it is a mark of that realm's respect and appreciation for your efforts. As long as a knight upholds the goals and aims of his realm, there is little reason to concern themselves with what outsiders think about the title.