Like Heather said, it is pretty much up to you. Here are some things that as a member who enters groups, I prefer --
-- Set the contest to start immediately. I hate to find a contest, read the rules, go to submit and then find it's not accepting entries yet.
-- I prefer contests to accept at least three entries per person, but not more then ten. Actually, I set all of mine to accept three.
-- I like to accept entries for five days, then allow voting for three.
-- I have changed my mind on this lately. I open voting to anyone. If the purpose of the contest is maximum exposure, limiting it to just FAA members seems silly. But I also explain why in the group rules.
-- Contest admins get to join the contests!
Just a few things I go by in my own contests. Your mileage may vary.