I did three hours yesterday and it wasn't too bad. Movies help. TV shows are better, especially comedies. I don't think I'd want to do more than four hours.
I can last longer on the trainer than the rollers (although I did do some three-hour roller sessions last winter) because I don't need to concentrate as much and can do other stuff like send emails and read bikeforums.
I'm curious to hear what others have to say about breaking up the workout.