There nothing anyone can rap about for 12 minutes that would make give a flying fuck.
7 minutes is my max. And thats only in extreme cases like Outkast. Most rappers making 6+ minute songs are just felating themselves the entire time and have no real reason for making the song that long besides the fact they got drunk in the booth and thought it would be cool.
ill agree that super long rap songs usually suck. unless they're made by Dälek, then they're all good.