I vote for twice a year. As someone above me said, a lot can happen in RP land in 6 months, and the FW events are great for playing around with different sorts of possibilities, particularly for muns who aren't in the game and want to give it a shot, even if just for a little while. But I also agree that they shouldn't be the only event/plot happening in May/November, and I like the idea of a big one that will be remembered by the characters acting as a larger plot.

Besides, it's not like they're compulsory. If you don't like it, don't participate. It's pretty simple, really.