I've always seen it play out like so -- 1st session at the Keep gathering info, talking to NPCs and making plans; 2nd session in the wilderness searching for the Caves of Chaos and/or dealing with one or more of the wilderness encounters; sessions 3+ dealing with the Caves of Chaos at an average rate of about 1 cave per session (not counting the ogre cave) -- some will take less, but some might take more (plus there's the other wilderness areas and more stuff at the Keep -- audience with the Castellan and such -- that are also likely to fill in at least odds and ends of a couple sessions). Plus the possibility of the party getting decimated (or TPK'd entirely) and having to go slow for a session or two breaking in new characters -- a very likely occurence in this module. if the party gets into diplomacy and roleplaying, either at the Keep or in the caves (or both) things could take even longer, as they could if the DM takes seriously the module's suggestions about monsters recruiting reinforcements and/or new inhabitants moving into previously cleared caves. The idea of this module being played through completely in 3 sessions, much less 1, is very surprising to me.