It sounds like you've bought the wrong games.
Try any of SpiderWeb Software's RPGs. They go on so long that I never get to the end of the demos. The only problem with that is I get bored and stop playing them. Then there's my game at ausgamedev.com. Its in the time space you mention (10-15 hrs), but you can replay it using different character types.
The question I ask about a game is did I have fun or was I bored? I'd rather enjoy a shorter game and have good memories, than be bored playing a longer forgetable game. As an example my all time favorite game is Fallout, which is kind of short for an RPG. Fallout 2 on the other hand was way too long and I got bored well before the end of it.
Perhaps the devs here can recommend some longer Indie games?