They use dissolvable stitches inside because otherwise they'd have to open you back up to remove the other stitches or staples, which doesn't make much sense. Generally 6 weeks is what they say to heal, both inside and out. Now your incision in your skin will be healed sooner than that, but it doesn't get as much pressure as your muscles do. Also it's entirely normal to have some numbness at the incision site because they've cut through nerves and they take longer to reconnect and heal.
You won't pass any stitches or staples, there's no foolproof way to 100% say that you've healed, but you'll be feeling better and better. You'll be able to move more easily, to get up and use your ab muscles without pain. Don't rush into doing too much, but as you get closer to the 6w mark you'll start to feel normal again and be fully healed. Plus at your 6w post partum check your OB will do an internal to see how everything is doing.
at 12:53 AM on May. 18, 2009