Most dungeons can be completed in 30 minutes. If you can't beat it in 2-3 times the amount of time that is the norm then there's probably something wrong with your group.

I did most of the story dungeons with an FC group without looking up any details of the fight. They're pretty exceptional players. We cleared most dungeons within 30 minutes or maybe a little more knowing nothing going into them. Some people are good at games like this, others aren't. If you feel a run isn't going quite as well as it should maybe you should just cut your losses and try for another group.