General Interpretation : Leveling up is a gradual process that occurs all the time PCs face challenging conditions. They train and practice all the time, but what makes them better is experience - that is what the XP system is about. Otherwise anyone could just train 24 hours a day for 2 years and become a 10th level fighter.
Rule #1 : PCs can level up in one of their existing classes at any time, it does not require weeks of training with a mentor. However, it does require down time to think and reflect - it cannot be done in mid-combat or in a perilous situation.
Rule #2 : Practically, leveling-up should be left to between game sessions as it can be distracting and time consuming. This means that adventures can be large and split over a number of sessions with leveling up between sessions. There would be no requirement for PCs to go back to town, level-up and then come back for more.
[EDIT] This does not stop us from allowing mid-game level-up where necessary. (e.g. previous session a PC gains not quite enough XP to level-up, he should prepare his leveled up character anyway between sessions and level-up when appropriate. So long as the DM is willing to count XP mid-game)
Rule #3 : Training costs. There are no GP costs associated with leveling up (except for conditions below).
Rule #4 : If a PC wants to multi-class, then gaining the 1st level in a new class does require 1-2 weeks training, and should have a cost of 1000gp. If the PC wants to gain a Prestige Class, then they will require to find a mentor and cover training and costs (2x that of an ordinary class).
Issue: a problem with all this is that the training costs were a general drain on the PCs gold reserves meaning that they did not get crazily rich. However there are other ways of handling this, e.g. paying to identify magical items (instead of the DM just telling them).
So, I welcome your comments:
Assif
