I chose once every 2 or 3 months. I use my gift cards and stuff to get stuff when I can, but since I got married it's a little hard to put money aside to order products all the time. I'm planning my next big purchase at the end of November.
I like to make a large purchase with a few items or one really expensive item about every two months but sometimes if a deal comes up I'll do it once a month. There was a 3 week period once where I made 4 separate purchases because things kept going on sale. It all really depends I guess.
Actually, the last time I made a legitimate purchase from this site was almost 3 years ago. I'm hoping to make another one at the end of this month or sometime next month. The money issue has been a real problem, though.