Just long enough.
You want to wait long enough for your body to recover. Get back to "fighting weight", and let some of the stretching recoil.
If you are planning more children
, wait until afterwards.
For the main areas: breasts - at least 3 months after the milk has dried up; belly - 6 months to a year, or until it stops improving on its own. You can always get a consult sooner for the best answer for you.