The National anthem for Qatar is 25 seconds long on average.
So I believe that that music can be any length, just because something is longer doesn't mean it is any better, you simply have more time to enjoy it.
Also I find that long peices of music (what I write at least) tends to have more looping areas, where maybe part of the song loops as one part plays something new. I try to seek as original music as I can find, so I have a fear of loops.