The guiding principle should be “little and often”, e.g. three minutes, four or five times a week (spaced practice is more effective than blocked practice).
You can see how much time your child is has been spending playing in games each day from your parental account:
From your family sign in page, sign into your parental account.
Click "Manage Users" in the main menu.
Select the name of the child you'd like to see statistics for.
Click the "Goal" tab.
Days coloured green in the calendar represent days on which your child met or exceeded the daily number of minutes set as the goal. Orange marks the days when your child played some games, however did not meet the daily goal. On days coloured red, your child did not complete any games.
To change the daily goal, edit the number of minutes that you require your child to play per day. You will see the colours change accordingly in the calendar below.
Press the "Save" button to set the daily goal for next time.
Please note that only time playing in Story or Challenge games counts towards the time played. Time spent in any of the other areas of NumBots (such as the Custom Shack) will not count towards their daily total. Therefore your child's time played will be less than their total screen time.
Also, your child must have completed a game in order for it to count towards their daily time played. Games are only saved once your child has finished it, so if your child exits a game halfway through, we will not have saved the game, therefore the time they spent playing that game will not be added to their daily total.