Stoneskin doesn't stay on it gets used up after a set amount of damage and the higher level dungeon you get the quicker that damage is taken. Tanks will use stoneskin up in a couple of seconds. Not worth casting it over and over. For some bosses it's worthwhile using it more than once on the tank but most of the time it's just to leave on the DPS so if they take a hit by accident it won't damage them much. Same with having it on yourself.