Its a better idea to make more than you need...never know when a backup will come in handy. Most people keep cultures for 30 days, by then production is usually slowing down. I keep mine after 30 days, but move them to another room (to avoid my nemesis...mites), they often continue to produce for another week or so (sometimes really well). Actually, today I had a large boom from a culture that was made on 11/24...hundreds of flies!
EDIT: DANG!!! That was a lot of responses at once...anyway, I agree with what Kris said, you should get at least one producing culture at the show.