With my first two, I was ready to do light to moderate exercise by 8 weeks. With my 3rd, I was still bleeding at 9 weeks postpartum, and occasionally getting the feeling like my uterus would fall out if I did too much activity. It really depends on the situation, and the best thing you can do is start slow and listen to your body. (Except in the first 4 weeks - you feel so energetic from not carrying the baby, but it is a LIE. You feel great so you go for a walk, only to suddenly realize you are NOT OKAY and have to sit down in the grass and wait for your husband to go home and get the car because your whole pelvic floor feels like it's going to collapse)