The prostagladins in the sperm help ripen the cervix at the end of pregnancy and unless your dr has advised you not to or you are in a high risk catergory then you dont need to worry!

Nipple stimulation can release oxytocin, and to naturally induce labour the time is 20 minutes or so manually or with a pump.

There is more info HERE about natural induction methods including what Ive mentioned!
Basically, no natural induction methods will work before your body is ready so go for it