How long is a song?

I struggle with song length. If you've said everything you want to say in six lines, should you even bother writing any more?

Of course there is this '3 minute pop song' idea, and 'radio play'. Well, all that 'radio friendly' thing aside - if you've said what you'd like to say, shouldn't you just duck out, 3 minutes or not?

OK, you may have to repeat yourself just so that the first time listener gets a chance to get your drift - but should you agonize trying to come up with something as good as your perfect six lines that you know you're never going to better? If you've 'said it all' perhaps a middle eight wouldn't so much as 'sum it all up' but prolong the misery of telling it again, but not as well?

Some songs were born short, and therein lies their beauty. Knowing when to stop is important. I've agonized over third versus and middle section for months before I've realized that the song was already finished - it was just shorter in lyrical length than my initial expectations.