How long does it take to train an unleveled person up to the point where he can qualify as a soldier, i.e. a combatant ready to be hired by the main character? Let's go find an answer.
Any kind of soldier is considered to be training toward the fighter class, so we can consider the training they do to be the basis of training any 1st level fighter would have received. 500 XP is required to make an unleveled character combat-trained, i.e. capable as a soldier, and then a further 1000 XP is needed to become a 1st-level in any class. (The combat training and XP-before-1st-level concepts are taken from Alexis.)
This means an unleveled character being trained into a first-level fighter requires 1500 XP total; the 500 needed to acquire combat training is thus 1/3 of that total.
The base age before factoring in character class in my game is 15 years old. The starting age of a fighter in my game is 15+1d4, so the average starting age of a figher is (16+19/2) or 17.5 years old. Thus, that the average fighter took 2.5 years to gain 1500 XP. (I've assumed negligible XP is gained while reaching age 15, but that would be an interesting rabbit hole to go down some other day.)
If combat training requires 1/3 of the XP below 1st level, and likewise 1/3 of that 2.5 year average training period, then we are left with a training period of of 10 months (assuming that the first 500 and the later 1000 XP are gained at the same rate, which you may contest.) So that's a good preliminary solution.
But we can go one better. With the fighter starting age distribution actually being 16-19, we could say that the training period is 1/3 of the actual training time, whether that's 1, 2, 3, or 4 years. The corresponding training periods are 4, 8, 12, and 16 months. What's the benefit of chunking things up? Well, given a gaggle of men freshly recruited, we can say that 25% are ready to serve as soldiers after 4 months, with another 25% being available each 4 months after that. Now we have some time-based figures we can use, should the party ever decide to wage war for themselves or take sides in someone else's conflict.
Ah, but we can go one better again! Who says we have to stop at 25% divisions? Let's divide the 16-19 distribution into 8 six-month periods, then we multiply by the 1/3 time figure for training, getting a two-month period. Divide your number of recruits by 8, and that's how many new soldiers you can turn out every two months. With 100 recruits, it'd be 12 (rounded from 12.5), at first the fastest learners, then the guys who took more time to become competent. Of course, this figure is assuming that we can train all 100 soldiers at once, but there ought to also be a limit on how many recruits a single trainer (leveled fighter with skills in training others) can handle at one time. Maybe you've got a 5th level trainer and he can only handle 50 recruits -- now that 12.5 soldiers-per-month rate is cut in half.
So there's a limitation on how long it takes to turn out ready soldiers, and a limitation on how many of your conscripts can be trained simultaneously, and these two limits combine with the starting number of recruits to determine, basically, how many guys you can add to your army every two months (or each 4 months, depending on the scheme we use.) You could even go further and figure out a monthly value, if that's the time scale you want to track things in. And, of course, if you don't want to use pure percentages, you could always roll 1d4 (multiply by 4) or 1d8 (multiply by 2) for each recruit to see how long it'll take to train him.
All of this training stuff can apply even if the person being trained is not as young as 15. I'm just ballparking here, but maybe normal training rules apply to men between ages 15 and 35, and it takes more time to train anyone younger or older than that age. I'm just speculating here.
Finally, just for fun, let's compare our derived figures to modern army training. It's no fair comparison, just something to think about.
In the US Army they spend 10 weeks on basic training, and then each soldier receives a variable length of additional training depending on their assigned specialty. Infantrymen do 14 weeks. That makes a total of 24 weeks, or 6 months, of training. Furthermore, this is a total of 6 months to train to what I would consider a 1st level fighter. Compare these figures to the average 10 months of training needed to get a combat-trained unleveled soldier, or the average 2.5 years needed to get a 1st level fighter. It's quite a difference, as it should be, since modern training methods are backed by centuries of improvement that a medieval or Renaissance trainer would obviously not have been able to use. I think the proportions between the army figures and my training periods are near enough to call it OK: 10 weeks for basic combat training is 1/4 of 10 months; 6 months for full skills is 1/5 of the 2.5-year average.
All I need now is a figure to determine how many people willing to be soldiers are present in each settlement, as a percentage of the overall population. Assuming they're getting their training from somewhere, since they're starting employment ready to go, I'd then be able to multiply that "number of willing soldiers" times the 2-month rate and get the maximum number of soldiers available for hire each 2 months. Looking to hire an army? Better go somewhere big.
OK, I've been typing this all day. Time to let it out onto the web.