Ah, okay, sorry sometimes I'm dense. :) This was at least the third one, I had run one a while back which rewarded three accounts, and then ran another which rewarded 20 and I left those 3 in as well. This one rewarded 50, and removed those previous 23.
I don't really have a set timeframe for doing this. I think it's "around quarterly" but I don't have dates set in my calendar. (Perhaps I should? Hmm...)