Right now I'm thinking one week if you're doing writing, and two weeks if you're doing art. Does this sound reasonable?
Bear the following things in mind:
-planning ahead. Unless you're one of the first couple of people to host you should have plenty of time beforehand and should know what you're adding to the journal
-you can also think about taping things in. Heck, you can tape in one page that unfolds to five if the final page limit is too hard to fit your section into.