If you are near labor and the milk is ready, then nipple stimulation will cause the body to produce the hormone oxytocin wich will start labor and increase contractions during labour.
From what I have read you have to use your palms in a circular motion over your nipple for 15 min every hour for it to do anything. I don't know about anyone else but I don't have time for all that lol. I know if my bf "plays" around with mine for any period of time I have super intense contractions for a half hour after.
I've ready that it takes 5 - 10 minutes if not more of continuous simulation to make a difference.
I've never tried it but to answer the question. Nipple stimulation is one of the many ways to achieve an orgasm. Orgasms can trigger labor.
I'm just curious as to how this is supposed to induce labor. I mean, if you have sex while pregnant I would think there's some nipple play in there somewhere, so how do so many women not end up going into labor early.