Personally, anything beyond once a day is excessive. My preference would generally be to only half two or three events a week. It's often enough to give people a bit to do, while still not being so frequent that their frequency makes them expected. It's the same with WoE: to me, if it was done every singe day, even if it was done with different sets of castles, WoE would be such a normality, that it doesn't have the same taste as if it's only done once or twice a week.
I've seen a server that essentially had an event day. Every Friday, they'd have a marathon of events to keep everyone nice and busy, often lasting two - four hours. It was a relatively small server, so the event holder could satisfy almost every event request, and it was a blast while I hung around it. Still, it does make it so that you had to be on at that time to get to see any events, so there are some negative points in doing something like that. =P