The proper amount of finish is a matter of personal preference as much as anything. We butchered our Angus and Hereford steers at about 14 months of age. I found it took Holsteins a lot longer to reach the same amount of finish. This is because they are bigger framed and slower to mature than the beef breeds. We butchered the Holsteins at 20 to 24 months or so. How fat they get will depend on how much grain they are fed. I put all my beeves on full feed, meaning all the grain and alfalfa hay they would eat, for some time before I butchered them. I don't remember now exactly how long that was. It wasn't economical the way I fed, but at the time I had access to cheap feed. Given enough time and enough feed, Holstein steers will put on a good fat cover. I had a neighbor who raised Holstein steers for beef. She kept hers for three years. By that time hers were nice and plump even though she kept hers primarily on good pasture and grain fed them only the last few weeks. It is kind of a case of you pays your money and you takes your choice.
I'm sorry I cannot be more helpful. It has been quite a few years since I last raised a beef and I have forgotten a lot. If I ever raise another, I will see if I can find a Jersey Angus cross calf.