How long does a cow take to reach maturity?
A cow is already mature. She is a female mature bovine that has had a calf.
However, a HEIFER will reach sexual maturity at 15 to 18 months of age. She will reach adult maturity at 3 to 4 years of age.
A cow is already at maturity because cows, whether they're beef or dairy, are female bovines that have had at least two calves and are at least 4 years of age, which is the age of maturity for a cow. So the question is pretty much pointless. But if you were to ask about a beef heifer or steer or calf that is such-and-such months of age, that's a different story.