Eh, what am I supposed to know?

In BtB 1E AD&D characters require 1-4 weeks of training to gain levels, determined by a subjective DM-assigned "performance rating." Low level characters generally require training/tutoring from a higher level NPC, but characters with good performance ratings can self-train at double the time (and money) cost. High level characters don't require tutors at all, but they still have to spend the time and money. This rule serves three purposes: 1) the performance rating system allows the DM to make a subjective judgment of player performance (in contrast to the objective BtB 1E XP calculus) -- a player who does a "bad job" (of roleplaying his class/race/alignment, or just generally) earns the same XP as the other characters but requires more training time (thus keeping him out of play longer and eating up more of his treasure -- and, if he doesn't have sufficient treasure, denying him further advancement as he's forced to go on zero-XP adventures to get enough money to train); 2) the (really pretty exorbitant -- 1500 g.p./week/level) training costs serve as a massive treasure-sink to keep PCs poor, hungry, and eager for new adventures; 3) the training rules also enforce character down-time, to keep the more active players from running roughshod over everyone else (remember that the default campaign-model in 1E wasn't a single "permanent party" but rather individuals playing in various combinations at their own pace -- see the example under "Time in the Campaign" for an example).
In my experience #1 was universally ignored (training was always calculated based on 1 week, and if the DM thought the player was performing badly he just gave him less XP), #2 was strictly enforced (but, as above, always based on 1 week), and #3 tended to be glossed over, since we always played under the "permanent party" model and either everybody trained at the same time or the rest of the party would wait around while the others were training. I never recall seeing a player/character excluded from an expedition because of training. The "NPC tutor" idea was also entirely glossed over/abstracted -- it was always just assumed that the character was able to find an appropriate tutor (or was self-training, though we didn't charge double time/money for that). Basically, every group I played in used the training rules as a treasure-sink and ignored all the rest. I can't speak for what everyone else was doing, of course.