I've successfully churned the UA card three times, with a minimum two years between each card, for 100K total I think. I've now done AA twice for 60K, but with more than three years between. I did the NW card twice, I think, and the the Delta card twice, but I received only half the offered miles on the second Delta card, probably because it was less than two years between the two.
I'm now waiting to see if Starwood will send me an offer for a second go-round.