I think a 15-20 minute time limit is reasonable for rooms with queues, with a 5-10 min waiting period before reentering. it would mix it up a lot to give those with slower connections a better chance against those who stay hours on end grabbing everything.
Personally, regarding these rooms, it would really be much nicer to have MORE rooms with multi/quick regeneration on shells!!! It's taking the fun out of the game since we don't have any other way to gain credits and items that are very expensive considering how little time we've had to play and the fact that there isn't a decent way to earn. 25 ppl in Adventureland Bazaar is too many. Every time I try to walk to a shell I end up clicking on a person's hat or arm that gets in the way and by the time I can close that to see where I'm going it's far too late.
I wish it was a common courtesy rule that people remove hats in the main shelling rooms, too.