Thinking about this I didn't realise how much I think about the limit button in a dungeon. I use it but only ever sparingly. Normally BLM so it's aoe,I used it once on a big pull and got moaned at for wasting the LB. now if I do that it's only ever at the request of the tank or healer.
As far as boss battles go I'll generally use it on the opening boss if it is full by then, though this isn't always the case.
On the final boss I'll always wait till t is about 15% hours, if there is Melee in the group I give them an extra 1-2% to see if they do it as I have this feeling melee does more dmg than my aoe? If they don't bite then I activate around 13%.
Dungeons like bardem or sohm al I just activate at the wing phase no questions asked.
Sometimes though you just have a good group and you all get into the swing of it and don't even realise you need it. Like a freight train we just plough through everything and before you know it you're on the final boss with it at 6% hp and there is no point, by time the animation and charge is over it'll be dead anyway. I just look a fool haha