I figure it all depends on how long you want to feed them and how fat they get. Really, we seldom let a steer get over 18 months old the Herefords are right at 1,000 lbs and the Angus and Baldies run close to 1200+. We finish for 90 days adding cracked corn to finish with 15# per day.
Alot of folks just finish on grass and ship on a predetermined schedule...we sell finished animals so each is a bit different and
size counts, we have some customers that don't want steaks that hang off a dinner plate and just about all want a leaner beef....we are a small operation so we can process our animals at different ages....for our own locker I finish a two year old Hereford on corn (20#/day) for 120 days and freechoice grass & alfalfa twice a day...not what my cardiologist reccommends but great beef.
Good luck to 'ya....Dave Mc