The raids have timers. The Binding Coil of Bahamut (which is the only raid-raid so far) has 90 minutes per turn. However, people usually use the majority of time and if they don't complete the turn in the time, leave and restart that turn. Which means raids can go on as long as people want. Some raiding groups only go for 1-2 hours, some 1-2 hours per day, and some 5+ hours per day, just depends on your raiding group.