I agree it would depend on the intensity your looking for. IF your looking for pin-ups then a week or two would be sufficient, but if your looking for, say, a splashpage type of art where people should be collaborating together then a month or two would be better.

It depends of the Type, if you wanna do a color contest, two weeks is good.
If will be a non- color drawing contest one 3 weeks as maximum
If will be a color drawing contes one month and a half as maximum

just opinions