People are different. Sometimes I have a burst of inspiration and can make a level in a very short time (I made "Heaven and Earth" in the space of about 6 hours tops, from concept to the first release). Other times I struggle with the rules and am not sure how to proceed, or just need a break due to creative burn-out or simply being too busy.
If there was a competition every month, then that would definitely be too frequent. Every three or four months, I think, would cause the next one to start too soon after the last one concludes (if it's even possible with the voting phase etc.), and yet every six months feels like an eternity. It's a difficult one.