If I remember rightly from the two 29 hour trips on a greyhound that I was forced to endure back in '03 (philadelphia to kansas city for anyone that cares), the bus would stop on average every four hours, for at least 15 minutes, in order to allow the herd a chance to stretch their legs, restock on supplies, have a smoke, and often, to change drivers. It was about the only thing that made it bearable..
Depending on the route and time of day, it can be a little cramped but the trick seems to be, take up two seats, then pretend as though you are fast asleep, if the bus isn't too full, people would rather find a different seat, than disturb a sleeping person.
It was... an experience to say the least.. never again though.