How much screen time should be allowed per day on mobile phone for 2-5 yr olds?

As much as possible, my friends try to limit the screen time for their children. Usually, they will only hand their phones over to their children for 10-15 minutes each time. While they never stipulate how frequently that happened in a day, they estimate around 4 to 5 times. Sometimes when they are preparing food for their children or when they are driving.