I would leave about 45 minutes, just in case there is a problem. You could have to wait for 10 minutes for the monorail in Epcot, then the 15 minute ride to TTC, then the 8-10 minute walk to the Polynesian. It's so much better to arrive early, and stressfree, than be sitting on the monorail, as it sits waiting for the tracks up ahead to clear, knowing you should be checking in at Kona Cafe in 3 minutes!! That's happened to me.