I think three full days would be a good amount of time. I toured that route in 2008, though from Vancouver north to Courtenay. My itinerary was:

1. Vancouver to Roberts Creek Provincial Park
2. Roberts Creek to Saltery Bay Provincial Park
3. Saltery Bay to Powell River, ferry to Comox/Courtenay

Some things to note about this route:

• You'll have THREE ferries: Courtenay/Comox to Powell River, Saltery Bay to Earls Cove, and Langdale to Horseshoe Bay. Saltery Bay/Earls Cove is on the Sunshine Coast itself, and has a frequency of 2 to 2 1/2 hours during the day. I remember waiting over an hour for this ferry. So timing is crucial.
• Like someone mentioned in one of your other threads, there are some hills on the Sunshine Coast. Nothing super-big by any means (especially compared to going over mountains).
• You don't see much of the coast itself, as the route goes inland. If you want to see more water, you'll need to take some side trips. If I were to do this trip again, I would definitely budget more time for that.

Hope this helps.