Good question. Initially mine could be measured in hours, (that was almost a decade ago), but now both last longer and are more debilitating. Was just thinking how I really can't remember a time when I was not in pain...yet at first it was reversed.
Mine tend to be triggered by stress, although there is also a 'rhythm' to my flares - roughly every two to three weeks I can count on a few days to a few weeks of brutal times, where all the cannabis/morphine I can take is still not enough. All I can do is curl up into a ball and try to will every muscle and joint to relax. Sleep is a big issue during my flares as well, as it is constantly interrupted by pain, have real trouble falling asleep, and endure colourful nightmares due to opiates. Towards the end of a a flare-up period my IBS symptoms remind me they are still around, as I struggle with various intestinal issues.
Not a lot of fun, is it ?!