My oby/gyn told me I would be ready by six weeks pp, but I wasn't ready for a while after. Even now (dd is 10 months), my incision area gets sore when I do too many crunches. I guess it's different for everyone.
I was comfortable doing situps less than a month postpartum this time around; my doc said, "whenever you feel comfortable," so that's what I went by. Everyone's body heals differently, so I'd go with my own energy & pain levels.
I was given the go ahead at 6 weeks but my body didn't feel ready for ab stuff. we walked tons but didn't start doing ab work till about 10 months and we started slowly. my babe is 4 months and its' just starting to feel as if everything is back in the right place inside.
After my first c-section, I could do ab work by about 8 weeks pp. Then I ended up with another section exactly two years later, and my ab muscles are shot. It's taking much longer to regain strength, and at 7 months pp I'm still not working the abs as much as I was used to.