Two stand out in my mind as being Really Long.
The shorter one was in a 3.5ized Return to White Plume Mountain; my (newly half-ogred) Int-8 melee cleric had time to get through a Maze spell while the half-orog Ninja Spy played follow-the-leader with a pair of lava golems, using a Staff of Frost to keep conjuring walls of ice on the floor to slow them down. I forget how long that was, but in the 20+ rounds area. The main tank actually had time to get back up from being knocked down.
The longer one in time was the super-long big fight with a beholder mage and a hag covey. My tiefling rogue first got Forcecaged, and then burned a Candle of Invocation to gate in a Planetar to help out. The beholder mage blanched, tried to banish the planetar, and instead managed to banish my rogue, and then gated in a fallen Solar to occupy the planetar's attention. The UMD rogue used both of his Defensive Rolls for the day to avoid death, too. We actually wound up the fight around, oh, 4:30 AM, and by the time we finished play, the sun was up. I have no idea how many rounds it was, but I'm thinking 20 or so, just really long rounds due to loopiness.
Brad