We keep them for as long as we can. 4 years seems to be the magic # though. They almost always seem to develop some sort of a problem by then. Either attitude, they turn into fence crawlers, they just don't test one spring, their feet go or they get sick. With 6 bulls we have no problems with father/daughter stuff regardless of how long we would keep the bull. We also run our bulls in separate pastures at breeding so it lessens the chance of injury, but even with that we are lucky it seems to get 4 years out of them.