It varies, largely depending on the level our characters are at. At low levels, you would be hard pressed to push a battle past 20 minutes, and at epic levels, a battle moving along at a good clip can easily take a couple of hours, since there is so much more that can happen, both for the player and the DM.
A lot of our battles take longer because the DMs in our group love to use strategy. The last battle we fought took about 2 hours and involved several near deaths for about half of the players. If it had been a straightforward battle in an open room, it would have been quick and easy, but we had hidden opponents, a room with obstacles we couldn't move freely around, and opponents who were aware of our presence and had strategically placed themselves to make it very hard to fight them. Add to that our almost total lack of spells since we had been going straight through this dungeon without a rest (we were trying to rescue a pregnant baroness who was about to be used as a sacrifice in a dark ritual. If we had rested to regain HPs and spells, they would have completed the ritual and killed the baroness.)