Buses start running, to the parks, by 6:30am, every morning. BUT they don't run quite as frequently as the regular park buses do. So, you need to figure that if you arrive at the bus stop, and the bus to Epcot is just leaving at 6:55, you may be waiting until close to 7:25ish for the next one. I have never waited more than 15 mins for one to come prior to 7:30am, but it's best to be prepared for a longer wait.
So...if I had an 8am time to be at Epcot??? I would be out at my resorts Epcot bus stop by 7am, no later than 7:15. I guess it also depends on what the 8:00 time is for...if it's an ADR at Akershus??? Need to be out at the bus stop by 6:45 in order to leave enough time to get through the park. An 8am Segway tour??? 7:15 will probably be early enough since you just have to enter the park and walk to SE area...about 3 mins.