I was thinking about playing some old-school D&D campaign with friends, and Temple of the Elemental Evil came to mind as it is an iconic and well-received module. However, I heard that it is pretty massive. What ballpark of size are we talking about here to complete the whole scenario? 10, 20, 50 sessions?
Its pretty massive - how many sessions depends on how long your sessions are, how efficient your party is at dealing with encounters and how you strike the balance between role-play, exploration and combat.
The Temple itself has 435 keyed locations with four attached "elemental nodes" of about 25-35 keyed locations. In addition, the moathouse contains 32 keyed locations, there are two villages, Hommlet and Nulb with 31 and 3 keyed locations each.
The back of the module says "... this product provides a complete campaign adventure which will take beginning characters from 1st all the way to 8th level and possibly beyond!" From memory, when I played I think we reached 10-12th level and spent several years of real time doing so. That said, we treated it as a combat grind - there are ways to by-pass whole chunks of the temple by using diplomacy and playing one temple faction off against the others.
Just crunching some estimated numbers in support of Dale M's answer:
Dale points out that the moathouse & temple together have 32 + 435 = 467 keyed areas (not counting the elemental nodes). Traditionally tournament games at the time assumed a 4-hour time slot, and up to 9 areas of content cleared in that time (e.g., see the AD&D A-series tournament play specifications). Based on that, we might estimate:
467 / 9 = 52 sessions (4 hours each).
Alternatively, given that the adventure advertises itself as covering levels 1 through 8, we might look at Gygax's estimate for level advancement in D&D:
It is reasonable to calculate that if a fair player takes part in 50 to 75 games in the course of a year he should acquire sufficient experience points to make him about 9th to 11th level... [The Strategic Review, Vol. 2, #2, p. 23, 1976]
This works out to an average of about 6 game sessions per level. So the leveling velocity suggests it might take:
8 × 6 = 48 sessions (4 hours each)
Thus, it seems like a rough estimate of about 50 games sessions is in the ballpark. Again, this ignores the elemental node areas; which all told might add another dozen sessions or more.
(The advancement rate quote was fresh in my mind because I recently blogged about that.)