Most people gain 1/2 - 1 inch per week. The closer you get to a 50 inch vertical those gains may slow down, but gains will still come steadily. You should also consider that some athletes can make significant gains through form, flexibility, strength and a variety of other factors. I have many athletes gain 2 – 4 inches through form alone! This being said, I stand by my guarantee, 10 inches in 12 weeks.
It can be difficult to accurately predict gains for an athlete even with a full assessment. Ability to recover from exercise, prior injury and exercise history, strength levels, plyometric and reactive ability, muscle type, body composition, hereditary factors, movement efficiency and form, and other factors help to make a more accurate prediction of the gains you can make.
You may also repeat the program to continue getting excellent results.