You do not need weekly one-on-one lessons to get better, and you do not need to be in the biggest program in town. What matters more is what happens between the sessions. I wish I could see my athletes five or seven days a week, because one hour out of the whole week is just not enough. If nothing is happening in the other six days, that hour can only do so much.
My answer is about five days a week of actual baseball. Practice means something different for every kid though. If he is a multisport athlete and he is running around catching footballs, that counts, and he does not need to do something baseball specific every day. But a player with a bat in his hand every day is going to perform better than one who does not, without the lessons and without the personalized coaching, because he is hitting whiffle balls and tennis balls and baseballs. He is just hitting.
I had an eight year old come in for one session, and I did not see him for two weeks. When he came back he looked like a totally different kid. I told his mom I did not know what he had been doing, but whatever it was, keep doing it. She said every day after school he asked to go hit, five or ten minutes off the tee, and she or his dad would flip him side toss. That was the whole thing.
A two hour team practice is not the same. Ten players in line for ground balls, ten pitches of BP and the next kid is up. That is reps, not instruction. The time that moves a player is a catch with dad in the backyard, hitting with a friend on an empty field, or a tee and a net out front.