Sometimes in just a few weeks they can learn to throw an effective breaking ball.
However, I get freshman in college that have never thrown a change-up. It takes at least 2 YEARS sometimes all 4 (in other cases 5
) for them to learn to throw a change up and feel comfortable with it. I really believe it is the most effective off speed pitch.
My point being, teach them good mechanics, and different grips of fastballs and changeups. It is also much like real estate, a strong focus on location, location, location.
One of the best ways for them to feel comfortable throwing the change up is to play catch and play longtoss by alternating with fastball grips and changeups grips. Longtoss forces them to use the same arm speed just to reach. Once they FEEL it, they'll know and start using it from the mound.
Let them develop thier elbow and shoulder strength before throwing curves.