I go with a range.
Depending upon a variety of variables—structure, style, lyrical content, whatever—I try to keep my songs between 2 and 5 minutes.
Any less than 2 minutes and it's really more of a sketch than anything. Much more than 5 and it starts to drag a bit.
That's not to say there aren't exceptions, but as a general rule, that's what I shoot for.
I also try to keep solos—be they guitar, sax or whatever—relatively short and to the point. Not a bit fan of long, epic solos. But, if that is a part of your style... go for it!