I don't watch porn every day. If I do watch porn to serve as my visual stimulus to get off to, it doesn't take particularly long. If I'm really, really into a particular girl and she shoots for the "big" studios, I might occasionally "use" a whole scene, from kissing all the way to popshot. So really, we're only talking 20-45 minutes maximum most days when I do watch porn.
Although.... if I'm horny as hell and have the house to myself and break a DVD out... I managed to last to the end of SCENE 2 of Belladonna A/K/A Filthy Whore once. I don't know how.