I think if the mega 14 level dungeon was part of a side-quest, then that would be OK. 14 levels would be rather long, but if there are way points/warps/teleport stones to get in and out quickly then it would be OK. If you get tired of dungeon crawling for a bit, you could warp out, and then come back where you left off later.
There has to be something to draw you there, and something to keep you going. If they are going with the Quest for XP instead of Kill for XP (which I prefer), then you would really need a reason to keep going besides loot and gore. But assuming that each level would take an hour to get through, then that would be 14 hours worth of content just for that dungeon. Like Brannart said, a game within a game. And I am all for that.
Another option would be to have certain levels that you can't access until your character reaches a certain level. Certain spells or skill levels to unlock the doors/puzzles, etc. Then as you level up, you would be curious to see what lies on the next levels of the dungeon.