When they can train. That means when they are done with the adventure, and out of danger, usually in a fortress or city.
My house rules on training to level up:
- 100 gp per week in expenses and fees. That doesn't include room & board.
- 1 week/ 2 character levels, up to max of 3 weeks. In other words, 1st to 2nd is 1 week, 100 gp. 2nd to 3rd or 3rd to 4th is 2 weeks, 200 gp. 4th to 5th is 3 weeks, 300 gp. It caps out there.
- Characters below 5th level need a trainer. The trainer must be the same as level as they are trying to achieve, or higher, in a related class on the arcane/divine/fighter/thief divide. In other words, a ranger can train with a fighter, ranger, paladin, or barbarian. After 5th level, they can train themselves. Generally, the trainer is a "real" detailed character, usually a retired PC who is now an NPC.
- I base the 5th level cut off on 'game feel' -- a mage who can cast Fireball is a 'mage of power' to us, so he don't need no stinking training -- and on the movie "Conan the Barbarian". I figure Conan was about 5th level when they let him go from the training school. After that, he leveled up at least 2 more times (down time in the city, down time with the MU) with just practicing by himself. Works for me.
- I let PC's barter for training. Recently, they turned in a big stash of arms they didn't want to some NPC's who were trying to equip troops, in exchange for training. I've never held back a PC from leveling for lack of money, but I don't let them blow it off either -- the usual answer for a poor PC is borrowing from other PC's. I like an incentive for role-playing and for needing gp's.