How long should you keep a bull with a cow?

It's best to keep a bull with a cow until a) the cow has conceived, or b) when she no longer goes back into heat. Usually it's best to leave a bull in with that cow for a couple months so that both her heat periods are covered and she has a higher chance of conceiving. But even then you never know. It's always best to check if she has been bred by getting her preg-checked or have a blood-sample done on her.