I would be fine with a 5 minute timer to re-enter if you have to pay some sort of toll or fulfill some kind of prerequisite to enter the dungeon, such as the beastmen seals in FFXI (de-aspected crystals?).

There is a great opportunity to get people to party or farm together to gain access to these dungeons each time, rather than just hand them out on a silver platter ESPECIALLY if there's a possibility of server congestion preventing so-many simultaneous instances.

Just not faction points, I hated that lol