I know it's kinda counter-intuitive, but the way I see the challenges going:
Lots of people start out strong at the beginning, but with a long challenge time, they're often too ambitious, and as time goes on, things come up, computers blow up, interest gets lost, and as less people keep the challenge going, I think the remaining people get less interested.

With a shorter time, people can't be overly ambitious and I have a feeling that the initial enthusiasm will carry forward long enough to reach the home stretch.