I've recently been introduced to Atmosphere, a hip-hop group with, this'll blow your mind, meaning in their songs. The average length of the songs is about maybe 4 minutes, but the thing is, they're composed in such a way that they drag on and on and can get so much musical and lyrical genius out in minutes.
But some songs can't, just because of the style and genre, because of the composure and instrumentation. A ten minute rock song is almost always pretty good, depending on the level of repetitiveness, and can also get out that same genius mentioned above (if it so chooses). But the way hip-hop songs are styled, heavy with percussion, a shit-ton of bass in the background, they may seem slower and to last longer just by being what they are. Some genres can't do that, also just by being what they are, so they have to go for actual length to simulate the same feel.
Here's a link to a page of Atmosphere's songs on youtube, if you're interested. I recommend you first listen to either Puppets, Guarantees, In Her Music Box, Smart Went Crazy, Scapegoat, or Yesterday before anything else: http://www.youtube.com/results?search_query=atmosphere&aq=f