Personally, I'd rather have them once a year, for reasons stated by others and some more. While fun at first, the event has become somewhat repetitive from my point of view, especially since I'm not its greatest fan in the first place (mostly due to lacking characters I'd like to throw around at people). However, I'd like to mention I'm actually pleased with the "bizarre dream" system rather than characters remembering everything, since, well, it's fourth wall, and some stuff happening during it is a little too weird even for my tastes -- but that way we have a choice of not even including the parts that would be potentially mind-breaking or whatever.

Moreover, I'm going to second (third?) what Kev mentioned: that I don't feel fourth wall should be treated as the "main" event, mostly because they don't really have much to do with premise and don't affect the game as a whole. It's more like taking a break from the actual game rather than contributing to it.

That being said, I won't complain if we choose to stick to two a year, either, although I think it would be nice to include something other than those in their respective months if that was to happen, even if it's just a simple swarm.