in a normal production system dairy cattle have a calf approximately every year. their gestation period is 9 months, just like humans. she will calve, start being milked, bred during her next heat (estrous cycle is about 21 days), milk for about 305 days and then be dry for about 2 months until she calves again.<br><br><br><br>
like kreeli says, as long as a cow (or a human) is being milked, either mechanically or by offspring, she will continue to lactate. but the amount of milk she produces will decrease significantly over time, so therefore dairy cows are bred to calve about once every year to keep them productive.