How quickly and how much muscle mass you gain depends on several factors, such as your diet, daily routine, genetics, and training experience.
The chart below shows how much muscle you're likely to gain:
In ... (time) you can build ... (muscle)
a month 1.5-2 pounds
three months 4.5-6 pounds
Six months 6-12 pounds
a year 20-25 pounds
2 years 30-37 pounds
5 years 38-48 pounds