When the calf is eating at least 2 pounds of dry calf feed a day. Personally, that's when I take it down to one bottle a day, but then, I'm a softie.
When my calves are eating about 2 lbs of feed per day, like I said, I knock out the evening bottle. I feed a bottle in the morning along with the calf feed for about 2, sometimes 3 weeks, then the poor babies are given nothing but the calf feed...with my heartfelt apologies.