Not long at all. Seriously. These are some of the shortest dungeons I've played in a Final Fantasy game ever. Even FF1's dungeons were longer and more arduous. Every single time I step into a dungeon in this game, I think "oh christ, this is going to be a long slog..." but in my experience this game requires VERY LITTLE leveling and the dungeons are not 3 hour slogs through Satan's asshole. The dungeon designs look pretty, but they're very straightforward and 'simplistic.'
I will say this, though: boss battles will fuck your day up. You can usually see them coming, thankfully, so if you're running low in spells, items, etc it's best to run back to town and go through it again. Once or twice I did fight a boss and die, which is frustrating because you feel as though you've lost progress, but as I said...the dungeons aren't that long.
PROTIP: shields are worthless in this game. Dual-wield like it's going out of style. See the game's logo?? That guy is dual wielding.
PROTIP: Trying to figure out how to crack the game's seemingly random job leveling system?? Well, it's not based on experience points, it's based on how many actions you take. So if you go back to the easier battles at the beginning of the game, have your party guard for 8 turns in a row, and then kill the enemy, you'll gain a level. Attacking, guarding, job specific abilities definitely count, but I'm not certain if using items or moving to front or back do. 8 may not be the exact number, but it seems to carry over from one battle to another.