One or two hours for a character, fully shaded and whatnot, and never over 8 hours for a fully background, fully shaded that sort of thing pic. I work fast. 'S probably just my style, but that's how I roll.
That said, I do know a few people who take a month or so to finish their big art pieces, those real masterpieces, and really you only get out what you put in. If I take any longer then 3 sessions on something it usually means I'm procrastinating.
It's a case of simply adjusting your patience to deal with the pressure of actually taking your time on pieces.