It would vary for the individual but 7 mins would be good. There are great groups that wrote great 3 min songs. I'm discussing what artists wrote with the complexity with layering and progression of song structures that are cohesive. Of course have an epic section like Deep Purple's " Child in Time"
Major artists that don't have an epic song
A song that.could start slow and build to an over the top finish or middle. There will be some arguments on this post. One artist that comes to mind is Bad Company. Fleetwood Mac. The Rolling Stones. The Yardbirds. Ok scratch that one because songs in that era were mainly short. Journey.