Maybe one week for submissions, and one week for voting. That way, one could also vote on the complete set of submissions on a fixed date.

@CharlieBrumbaugh I enjoy it the way it is and think I might not participate as regularly if they are more often because it takes me significant time to find and select the photos, but this is only my opinion and not necessarily a reason not to do it. Also most people will likely try to submit as early as possible to get more votes, so naturally there might be less participation later on.