Rod,
I have spent over 3 hours on a bike without stopping and I could do more. However, doing 2 hours straight on a bike is probably enough. Most organized centuries I have seen have SAG stops every 20 miles or so. Two hours or a bit over it will get you to every other SAG stop. Whether you are on an organized ride or not it is good to stop and air out one's hinder parts and get the contact points off of the bike. As time passes things get more painful. Also I have noticed that a short stop prevents things from developing into aches and pains that might last for a few days. Besides, no one has ever paid me to ride a distance in a certain time so whether I get there in 4 hours or 4 hours and 10 minutes makes no difference.