Only about 15-20 mins, depending on which resort you're headed to. The boat takes 25 mins, from DHS to BW. It stops at the S/D, then YC/BC, then BW and heads to Epcot.
If the boat is there when you get to the dock, fine, get on. But, if there is no boat at the dock, or in sight, then you may be looking at a waittime of about 15 or so mins. But, if you're tired, it's easier to stand and wait for the boat than to walk along the walkway back to the resorts.