As often as the membership will bear!
Seriously, I think it's a great thing to do once or twice a year. Certainly seems appropriate to do two for steelhead due to the split in the species et. al.
It's a bit of work to do the swap but the fly swaps I've participated in over the years have been wonderful and I treasure many of the flies I've received.
If too many are going on it kind of gets overbearing, so it's best to keep them few and fun.