It's been quite a while since I've driven some of those routes, so this may be a little inaccurate.
Camps Bay to Stellenbosch will probably be about an hour and a half.
Stellenbosch to Montagu about 2 hours.
Montagu to Oudtshoorn abouth 3 1/2 to 4 hours.
I've never driven Oudtshoorn to Wilderness. I'd guess a couple of hours.
Wilderness to Knysna is about half an hour
Knysna to Plett is a little under an hour
Plett to PE is about 3 hours
Those times are from when I was living there, and knew the roads reasonably well.
This is quite a useful site for the distance you'll be driving around the region.
If you have the time, try to do the trip to Featherbed while you're in Knysna.