I think the perfect amount of time for a contest is 1 month. I use about that much time to work on my entry and physically I couldn't do more.

I work 10 hours a day - spend 3 hours with family, sleep 6 - leaving 3 per day for the contest and relaxing - I cant manage more than a month of spending those 3 hours everyday on my entry

Contests that are 2 or 3 days are fine in that they test what you know - longer contests allow you to learn while you do your entry. I built a wonderful framework in my game and it will manage changes with hardly a blink. (I am busy adding computer AI as my son keeps asking me to play - and 10 games a day is a bit much)