I'd say anything over 5-6 minutes is long. I'm referring to whatever's popular, be it rock, R&B, whatever. Prog/rock "Epics" are something else altogether. I remember when "Light My Fire" was a big hit in 1967 (or whenever it was). I think the song originally was about 6-7 minutes long (I'm not really a Doors fan so I don't remember the exact length of the song). It was edited for radio to over 5 minutes, which would have been considered "epic length" on AM radio back then. It seemed really long. That organ solo went on for ever.