I take breaks as soon as I start to notice a diminished ability to hear the difference in frequencies, usually for about half hour every 2 - 3 hours. Remember that 85 decibels for 8 hours is roughly equivalent to 110 decibels for about 2 minutes. You can still get some hearing damage at 80db and above, even if it takes a long time. Remember that the longer the exposure, the longer recovery time you need, so it's better to take breaks more frequently than have to stop work for several hours after a long mixing session.
Like Colin said, if you're dealing with clients, pop in some EQd ear plugs. It's a great investment and you can get them in several db levels. Usually the -15 db have better EQ responses than -10 db or less. Most people want the sound cranked up because "it sounds better that way."
There should probably be another question that covers standard mixing levels and room calibration for film, tv, video games, etc.