In january (off-peak) I got to Epcot at 6:30pm, walked to World Showcase, around the lagoon, stopping to meet Beast in france, Pooh in UK, Aladdin and Genie in Morocco, rode Mealstrom in Norway, El Rio del Tempo in Mexico, had sushi in japan, kids ate in USA and had a cake in France and watched some of the bellydancer in morocco. these are the only countries i stopped at on my way round and not in much detail. We made it out to the lagoonside by 8:45pm to wait for Illuminations. So a small tour can be done in 2 to 3 hours if you were briefly stopping at each pavillion.
However i loved it so much and felt sad for cramming it in like that, so i am going back next October just to spend more time appreciating the gardens and step inside some of the shops which I didn't get to do at all this time.
I estimate that if World showcase opens at 11am, you could be done by 2:30pm latest and in MGM by 3:30 latest (allowing transit/ entry time). this would give you a little time to hit a couple of attractions. be careful with little ones though, they may not last this kind of day, unless they nap in a stroller.
have fun,
Claire xx