Duncan...think you probably asked 2 questions in one. "Lifespan" of a bull (bovine) probably depends on the breed. Longhorns can live into their teens and beyond. "Fertility" (lifespan) again depends on the breed, condition of the bull, libido, sperm count/motility/viability. "Keeping" (lifespan) of a bull depends on how many heifers & cows you want him to service without getting into "inbreeding" which can accentuate heredity defects if closely bred (to his dam or daughter). If you don't want to go to the expense and bother of owning a bull or taking some risks with a leased bull, you might want to consider artificial insemination using semen from a top-quality sire. If you have a top quality bull and he has serviced all of your second-generation (prior and future) heifers and cows you might want to keep him and let him rest or lease him out to others. Otherwise, it might be cheaper in the longrun to just purchase semen and AI your females.