3 months. If I'm gonna enter (which depends on when it starts, because I'm wrestling finals right now) I'm gonna need lots of time to work on my entry, which will probably look awful in the end anyway. xD
Very often I have large gaps of regular activity on-line, as a result I often learn of contests too close to the deadline to plan anything. The longer the contest runs the better my odds of finding out about it with plenty of time to participate.