Well, I'll end up doing what people want. So far it's 2 for 6 months, 2 for 4 months, and 2 who didn't state a preference.
Although I don't plan to participate I should note that I've kind of stopped taking progress pictures / measurements / etc. since I dropped out of the last challenge. There is something about having a specific date/time to report these things that tends to keep people in line.
I do think that the January challenges do tend to attract a larger percentage of 'resolution' types. I notice we get a fair number of people in at that time who may not even stick around and post starting pictures. I think I need to get back to monthly pictures myself, even if I'm not posting them.