Drop Grit when you face a boss or add(s) that is noticeably doing low damage to you or if healer keeps healing you at 99% HP. Just play around with it. When there's one add left turn off Grit and if you can manage more go for it later on in the dungeon, but pull first with Grit on. (Assuming you know how to build up MP and do high DPS while keeping control of the aggro)

gather up all the aggro from your pull, drop Grit, pop some cool downs (depending damage taking with grit on/off) and explode with DPS while building up you MP for the next pull

example: The first boss of The Wanderer's Palace (hard) currently I'm ILVL 231. I dropped grit around 60% after noticing healer was having no problems healing me and my HP stayed above 90%. I dropped Grit and DPS'ed while watching aggro. Sometimes i can only keep grit off for a few seconds so i don't bother because of high lvl DPS player taking the aggro fast. Same thing applies with adds.