It varies, has been my experience. With my first, my OB technically cleared me at 6 mo PP, though he knew we had not exactly been preventing anything. I got pg at 11 mo PP and had a normal pregnancy (c/s births, too).
I had some losses and fertility issues after #2 was born.
FIVE YEARS later, I completed my 3rd fullterm pg. I am currently 4 mo PP and would love to have one more, though with my history, I don't know how likely it will be. I have not yet been given the OK. I had some scar tissue and a complicated delivery...