I didn't start working out until 4 - 5 months after. Then I gradually eased into exercise and began upping the intensity after each month. It took 13 months for me to reach pre-preg WEIGHT but I am not pre-preg SIZE!!! My stomach is bigger at the same weight so I need to tone the muscles still!
edit: I should add, 13 months after giving birth I reached pre-preg size (not 13 months after starting exercise). I should also mention I gained 37lbs from the pregnancy. So I lost it all in just over a year.