0 nanoseconds. There is always a risk when your attention is diverted from the outside world when you control a vehicle.
From a consumer's point of view:
Give me physical checks for basic tasks. I always want basic navigation buttons directly on the steering wheel to control navigation through screens, volume and search, etc. I want dedicated buttons for the car settings such as cruise control, speed limiter which will often have to be set and reset. I want physical climate controls to be able to adjust the temperature and activate the heated windshield and fan settings while feeling lonely, especially if the screen fogs up while driving.
Give me voice commands for less used tasks or tasks that require more configuration. If I have to call someone or set a destination on the navigation system, I don't want to mess with the touch screens while driving and I prefer not to have to stop every time.
Give me options that can be browsed using a selector wheel (like those from BMW, Mercedes, etc.) or directional buttons on the steering wheel so that I don't have to try to press a button on a screen while the car is moving. I find it a terrible experience as it requires significant concentration in order to be able to touch small tactile targets while trying to stabilize your hand with the movement of the vehicle.
Most functions should be easily controlled with both hands on the steering wheel and preferably with a minimum focus required to watch a screen, so a head-up display or a display in the instrument cluster helps a lot.
I am of the opinion that touch input should only be used as a backup. If voice recognition fails and a user has to manually enter their input, they can use an on-screen keyboard. If they are stopped and want to go directly to something on the screen or quickly scroll through a list, they can use touch rather than moving the cursor with the buttons. Use touch to improve the experience, not to replace other input methods.