Most of our combats are over really quickly in about three or four rounds at the most with the occasional seven to ten round combat. However, we just finished Heart of Nightfang spire and the climactic battle took us a whopping 36 rounds.
Possible spoilers although the module was heavily modified
The first 20 or so rounds we did really well and didn't take a single point of damage (thanks to lots of death wards and neutralize poison) taking out lots of hags, spectres, shadows, bodaks and wraiths. Once we got to the vampires, the BBEG and the undead assassin he had summoned out of the opened Banewarrens (about our fifth confrontation with them) it got a lot more difficult with everyone coming close to death several times. At one point my eldritch knight got into a spell duel with the BBEG mystic theurge who had an antilife shell going, it was a lot of fun. First time we've seen a successful counterspell (he dispell countered one of my disintegrates). Also the most dispels thrown, we through four and used daylight to counter 3.0 darkness while he had two unhallowed dispels we had to go through and threw six at us, I got targeted four times. Longest combat we've ever had in 3.5.