A Tyrannosaurus rex growth curve is S-shaped, with juveniles remaining under 1,800 kg (4,000 lb) until approximately 14 years of age, when body size began to increase dramatically. During this rapid growth phase, a young Tyrannosaurus rex would gain an average of 600 kg (1,300 lb) a year for the next four years.
That means that for four years a T. Rex gained about 3.5 pounds a day. Which is about 25 pounds a week. Which is about 100 pounds a month.