AS long as you are also incorporating some long toss, tubing, and some mound work you should see some big strides. You will not see them in a week or so.
I waited until my son turned 13 and he worked his butt off. His Velo went from 68 to 73/74. He also was on a long toss program. He threw the underweight balls into a net.
Also did backwards chaining using the Ron Wolforth DVD's and lots of running.
Hardwork really paid off as he is playing multi positions due to a strong arm P/C/OF. If you have a strong arm and can hit they will find a place on the field for you!