Adding to what Nova said, the key to successful feeding is getting the calf to peek at milk, they need a minimum of whether real milk or real all milk replacer of 20 20 20,, 12 pints which is 6 litres a day. You can break in two or 3 feedings, 3 is best but two is more convenient. Once they peek , should be around 5-6 weeks old, ensure that they are eating lot's and lot's of really good calf starter ration. Once they are eating and getting their protien and energy from other sources you can gradually cut back on milk.
I do not and will not ever ween before 2 months as the rumen is not fully developed and to have the best BF operation start cutting back at 2 months a pint a week and when they are on 2 pints or less a day for a week you can stop all together. It is also the least stressful way to ween a calf.
Once weened ensure that their feed is as good as they had access too or better while BF'ing and keep it going ,if you do not wish to supplement the little ones with grain and really good hay then you must feed longer, 5-6 months minimum, just like the cow would do.