First kid - when I weaned. Or, more accurately, around 37 weeks when I thought, "Okay, good enough. You can do anything you want to me if it will help get this baby OUT!" But I'm not sure that's really the right kind of desire, just accepting that sex will suck less than being super pregnant for a long time.
With this second pregnancy, I did get my sex drive back with a vengeance around 15 weeks, but I'm 27 weeks now and even though I have the desire still, it's starting to get complicated again (belly, carpal tunnel, sciatica). Poor DH, who is always great about it, probably can't wait for the "get baby out" sex in ten weeks!