Once a month on AAdvantage cards....
Just got a Biz class ticket from ELP-HKG on AA/JL for Mar 2007 without flying one mile...total cost 110,000miles
Cost me $1550 in spending (which I was going to do anyway on Chairman card) -- so it has worked out so far...
I have planned on hitting AA lifetime gold by 35 (am 30 now) and lifetime plat by 40...just by churning...