When I start off, I play for 5mins on 3* {gets boring otherwise}. Then when I progress to 4*, I play some for 5mins and some for 10mins. Then When I have completed most things and feel I need a challenge to push me to the finish, I play for 10mins on 5*.
After that, when Ive got everything and think that the games more or less complete in terms of editing, then I play 5* for 30mins. Quite good fun. Usually ends up 4-4.