This may work for something like Steam, where they can track the hours you've played to see if yoiu were really just "testing the waters", but what about retail purchases that do not use Steamworks as their DRM?
Indeed, that's why I specified an account-based distribution service. Otherwise, I'm not sure what the answer is - this is the proverbial grey area and obviously some people would abuse it.
There is also the issue that some games, though quite good, do not contain a lot of hours of content. What is a fair measure of "trying to see if you like it"?
I guess this isn't so hard to figure out. A couple of examples.
For a single player game with a linear story line such as Half-Life 2* it will be split into chapters or sections. The developer will know how much they want to allow the game to be played and could allow only the first two or three chapters/sections to be played while allowing a refund. When this point is reached, an alert could pop up warning that progress beyond this point will forfeit the refund. Also, a time-out of say, two weeks would be reasonable in which to decide.
For games that just have a deathmatch mode perhaps, like the Unreal Tournament series, perhaps they could be time limited and/or limited to certain maps and game modes.
There's lots of variables and possibilities that could be adjusted here and these are just two possibilities.
The descriptions above sound very much like demo games, don't they - and they are. I think we all remember a while back when the big games houses said they would stop doing demos, because it was "very expensive" to make them. What BS. They're just greedy and don't want their games to be scrutinized before purchase, in case people realize what a crock their buying into. No, what should happen, is the return of free demos, like we used to get. If one can have a decent free demo, then it becomes reasonable not to allow refunds. One modern game that does have a demo mode is Bulletstorm - and more power to the developer.
*Ancient example lol, but it's my favourite linear shooter of all time.