My previous raid group had a 10 min. break halfway through.
My current raid group doesn't really have a set break time, because people tend to go afk whenever they need to smoke/bio/whatever. Sometimes there might be a 5 min. break inserted though.
Most common break for me has been 5 minutes. Current group I run with does a 10 min sometime in the middle of our 4 hours on Saturday. People get up for other things from time to time during the raid but it's usually restricted to trash pulls so we can make as much progress as possible with 1 day a week.