There isn't too much of a simple answer to your question. In terms fo role contests, I like to keep a general schedule of every couple months (plus or minus a week or two). I don't have a specific date because of the way I run the contest, basically using the time stamp to judge only roles entered prior to announcing--if my schedule was published that wouldn't work.

There is also the aspect of getting others involved. In the last role contest, Kastellyn was kind enough to take part, which meant finding a start date & closing date that fit both our schedules.

In short, I'd say look for the next one 2-3months after the last, though if you regularly write and update the roles on your mortal, you will hopefully not miss all of them.