At least one full period cycle, two if possible.
My doctor told me 2-3 months, but I have also heard people say to just wait until you have one normal period.
I was on the BCP for 13 years and got pregnant the first cycle off the pill. I had a very very easy pregnancy and a very healthy baby boy who is turning 1 next week. I always had regular periods and my due date was right on. Do what is best for you :)
My doctor told me they prefer 3 full cycles, but she also said when you stop the pill start taking a prenatal vitamin (even if you are waiting the 3 full cycles) and use a condom.