The AD&D game is intentionally very flexible concerning how slowly or quickly characters earn experience—in general, this is left to the discretion of the DM. Some players prefer a game of slow advancement, allowing them time to develop and explore imaginary personalities. Other players like a much faster pace and a definite feeling of progress. Each DM and his players will likely settle into a pace that best suits their group, without even realizing it.
There is only one hard and fast rule concerning advancement. Player characters should never advance more than one level per time experience is awarded. If a gaming session ends and a character has earned enough experience points to advance two levels, the excess points are lost. The DM should give the character enough experience to place him somewhere between halfway and one point below the next highest level.
An average pace in an AD&D game campaign is considered to be three to six adventures per level, with more time per level as the characters reach higher levels. However, it is possible to advance as quickly as one level per adventure or as slowly as 10 or more adventures per level. The DM should listen to his players.
If the players are enjoying themselves and aren't complaining about "not getting anywhere," then things are fine. If, on the other hand, they grouse about how they never get any better or they're quickly reaching the highest levels in the game, the pace of advancement probably needs to be adjusted. This, like much that deals with awarding experience, may not come to a DM immediately. Let experience be your guide.