Dun said "If you don't retain any heifers sired by your bull you could keep him till the year before he doesn't get the job done anymore." That answer is exactly correct!
Buying and selling bulls is like buying and selling stocks. It sure is hard to let go of a winner but that's when you need to sell them. We buy young bulls when they are just tall enough to do business. We generally sell them by the time they reach five years old, maybe six. After that they usually decline in virility and burn too much grass for what they get accomplished.
Craig-TX