You have to pick your mark.
I've never done it right off the bat, but definitely within five minutes of meeting a chick and gauging how drunk/slutty/horny she is.
Success rates vary but if you can pick up a woman's "fvck me" signals, just go for it.
And it's all in the delivery. If you can put it out there with a healthy twinkle on your eye and a big grin, you'll stand more of a chance.