If you haven't used any type of natural method for birth control (NFP or FAM) then I would say not to rely on it. You should know exactly what signs to look for that will indicate when you are fertile again. I have seen a lot of women who even EBF who get thier period back at 6-8 weeks PP. Here's more information on it:
http://www.kellymom.com/bf/normal/fertility.html