Grainger
Explorer
It's two years of real time. In-game, the whole thing could take place over a month or two. That's something I came to realize while playing in Pathfinder, with their official Adventure Paths.
If that sort of thing seems off to you, then I highly recommend that you require training time of at least a week between levels. It's not the sort of thing that would work in a fast-paced adventure arc, but it should help solve the problem of why the world isn't flooded with high-level characters.
Thanks for the tip - I forgot that game time can go even more quickly when I wrote my post! I've actually implemented the Slow Healing official variant rule for a couple of reasons, but one of them is to slow the campaign down. I don't like the idea of training between levels, really (it's just more admin, and I also like the idea of characters sometimes levelling up when far from civilisation)... I'll just give out XP more slowly, and see how that goes. I might go down the training route if it still seems too fast.