How often can a 12 yo practice baseball long-toss

As long as it takes for the little punk to get it right and start throwing like a man

at least 3 times a week. Strengthens his arm especially if he is a pitcher or outfielder. Make sue he has proper form

Long toss is good. It helps them strengthen their arms to prevent injury. A big part of it is listening to your arm. I would say 5 times a week. That's what I did. But if you feel any pain at all shut it down and rest accordingly. Also running helps to break up and loosen the arm. Hence why a lot of pitchers run after they threw a game. Also stretching the arm out and moving slowly to long distances is key.