How many times should we focus on ball mastery skills?

To be short, as much as possible! It is a universal law that the more we practise something, the better we get at it.

5 to 10 minutes on training, during warm-up before a game and on other days at home.

The more the better, with an emphasis on frequency, not duration. Better 10 minutes every day than 1 hour a week.

Why? Everytime a player practises ball control drills he goes trough a cycle of loading the corresponding procedures out of long-term memory while the new improved procedures will be processed and stored to memory during the next sleep. The more cycles we do, the better it will stick!

Never introduce too many new moves at once because the procedures the central nervous system uses to fire the required signals to the motor-units in the muscles can easily get intermingled which could stall the progression. Recently picked up moves are also still fragile and vulnerable to get mixed up. As a rule of thumb it's better to focus on 1 to max 3 moves at once until they can be executed proficiently.

As soon a player masters a whole collection of drills and moves, they should be repeated every now and then. So, it is a good idea to introduce scheduled repeating sessions.

Again, the more cycles we do of loading, processing and storing the required procedures in memory, the better it sticks!