We always go by that little ball of fat that appears under the chin. Usually, with British cattle, anywhere from 80 to 110 days on full feed. If feeding grass/hay and feed combined, it will take longer of course. When you can see a soft ball of fat underneath the animal's chin, he's ready for the freezer. This is assuming you started with a weaned calf of 8-10 months old. A younger calf will grow longer before fattening.