I think it does vary. I guess the only rule docs follow is that if it happens before 37 weeks, it's premature labor so they usually try to stop it. Anytime after that is okay. I hit 37 weeks tomorrow, and I'm definitely starting to feel more pressure and cramps, but nothing consistent that indicates true labor.
I wish there was a direct answer to give you, however it varies for each woman
your body will know when the time is right and things will start to happen.