Pitching: how much to practice?

Whoa Whoa People. They are 9 and 12, let them be kids.

Wait till the kids are older to use weighted balls and all that, they are not physically structured to en dour that stuff yet. Work at different lengths for speed and endurance, keep things simple for them to understand. Try push ups, and sit ups to work core muscles and that will help. But make it fun and not work or you will lose them.

You need to work one day at a time, you start talking about 100,000 pitches before varsity ball you are getting way ahead of yourself. DON'T BURN THEM OUT. Remember this is their life not yours. Make your practice serious but fun and they will want to do it and before you know it you will have 250 with warm-ups done and they will have enjoyed themselves as well.

100,000 pitches before varsity ball seems a little excessive to me. That might depend on how hard you were throwing every practice. If a kid is really cutting loose every throw they're going to have arm trouble or burn out.
I didn't really get serious about pitching until my mid teens and by the my early twenties was pitching four nights a week with no problems. But everybody is different, you have to know your own body, what works for one doesn't always work for another. Strong legs are important. Natural ability is also half the battle whether we want to admit it or not.

My daughter just turned 13 a month ago. My goal is 400 - 500 pitches per week over three or four days. She's been pitching in LL for three years and this is her first year doing travel on a 12U b team.

Typically after having a regular catch for 5 - 10 minutes I have her do the following:
10- 20 arm whip drills with feet stationary (feet, hips & shoulder about a 45 degree angle)
10- 20 arm rotation drills with feet stationary (feet, hips & shoulder about a 45 degree angle)
10- 20 arm rotation drills while taking a short stride & foot drag (feet, hips & shoulder about a 45 degree angle)

Next we may incorporate another 20 or so pitches working another drill

Next I have her pitch to each corner while I catch until she hits a certain number (5 - 10 each corner). For now she's hitting around 50 - 60 percent so the total usually is 50 - 80 pitches.
Finally we work on change-up or drop for 20 - 30 pitches.

This seems to work well enough so that she's improving while not burning out.

ttt

my daughter is 9yrs old and throws twice a week, about 100-150 pitches a week max. She has been pitching for 6 months now. We just pitch fastballs and some changeups, thats it. I am trying to get her mechanics sound and I want her to have command of the strike zone. All the rest can come later if and when she shows the desire to play, I don't want her to get burned out.
TM

9-12 year olds... agree 3 nights, MAYBE 4 if they want to and are having fun. older girls - you never know what their attitudes may be or how they change based on school, family situation, boyfriends, etc.
----Heavy days alternating with light days... one thing is for sure..... every girls is different and you can't compare situations becuase too many variables..... a players drive, pitching coach?, parents support or pressure?, is it all work or some fun? some competiveness? Are they getting excited with their own improvements? Motivation to WANT to practice to get even better??
- ok, pitching practice should NOT be all throwing the ball to a pitcher!
Mechanics of their stride, Leg drive, Arm circles, wrists-lots of rotation on the ball, LOCATION, I like to use the pitcher's break time for a couple minutes of talking about the mental part of the game (composure, how does she act when an infielder makes an error, situations - when to throw and NOT to throw specific pitches, what is intent of each pitch, etc... you'd be surprised that if you teach them a little at a time each pitching practice, all of a sudden you have a smart pitcher who thinks for herself!), I also like to mix in Pitcher's fielding work in (aligning their feet/shoulders to the lead runner, stepping to their throws to 1B, fielding bunts and throws to each base, throws to home and shovels to home, breaking towards 1B on balls hit to her left side, breaking across te line when runner on and ball to OF, NOT FIELDING high popups even though they often are some of our best athletes, but often cause confusion between 2 players, covering home on passed balls, NOT Reaching and deflecting balls to our middles, etc.)
*** Breaks up the monotony of just throwing 15-200+ balls in a night.
Alternating heavy and speed drills and light days of THROWING-mechanical drills, work her breaking balls, little location, fielding, sometimes I'll have them pitch 3+ innings with situations I give them to put pressure on in practice so come games, she's to some degree lived the situation and pitched through it successfully) means you could/should at higher levels be throwing 5-6 days a week, again NOT always heavy, but planned for heavy and easier days/maybe pitching w/ batters in the box!.