Short songs - anything upto 3mins
Long songs - anything over 6mins.

Myself, I prefer long songs. They need more attention. Some songs can drag on a bit if the band haven't done enough to make it interesting e.g. Wilco and Mars Volta in their 'White Noise' phase. Don't get me wrong, I do still like short songs - Futureheads' debut is still one of my favourite albums.
Which do you prefer?