I like @johnbucket's idea. We could say the date when the subject will be released (say, a Saturday afternoon) and people would already know that, from that specific moment on, the challenge begins. People will have time to schedule and will not start composing without the subject.
I've composed fugues in a day. It's totally ok. I'm in! o/
(I am totally in the mood of composing fugues, but I can't come with a convincing subject... maybe this challenge is everything I need)