Are you talking about T1 Village of Homlett or the full temple? T1 can be done in a couple sessions.
The Temple of Elemental Evil is practically a mega dungeon. I've seen entire campaigns centered around it.
It can take quite a while. The dungeon tends to be very 'meat grindery' at points. Lots of running in and out to heal up. At the end of the day ToEE is probably one of the biggest modules produced by TSR so it's a pretty big undertaking to DM.
At the end of the day I wouldn't look at it in terms of 'sessions' but rather months or years of playtime.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.