How much weight must a holstein gain before she is a milking cow?

A holstein heifer typically must be about 60% of the cow herd's average mature weight; so, if the average weight of all dairy cows in the herd is 1400 lbs, then she has to be around 840 lbs or more to conceive and be pregnant. But by the time she is fully mature body weight doesn't matter much; it is the fact that if she is bred and has had the calf that she can officially be a milking cow.