t4tutor,
Great question with no "set" answer. Everyone is different. Generally, we say that it takes 21 full practice sessions to build or change a habit. For folks that shoot 3 times a week, that would be in the area of 2 months and concentrating on correct execution for each shot. NOT really the number of arrows but reinforcing the new "habit" correctly EVERY practice session and EVERY shot.
In the beginning, most shooters that start truly engaging the back, will find that the length of the practice session is pretty short and you will fatigue and "feel" it the next day. As you get better with it, your sessions will be longer and you will find that using the system will allow shooting heavier bows AFTER you learn.
DO NOT shoot at a bull's eye target in the beginning, only at a "blank bale" to catch your arrows. Concentrate on WHAT you are doing and NOT on hitting anything.
Hope that helps!
Arne