How many months until a cow is ready to be slaughtered?

Since a cow is a mature female bovine, she has the potential to always be "ready" to be slaughtered if she's culled from the herd for a certain reason.

Of course, others state that cows are never ready to be slaughtered, but that's based on human opinion. We humans never know if a cow is ever "ready" to be slaughtered or not.