0 nanoseconds. There is always a risk that your attention will be distracted from the outside world when you are driving a vehicle.
From a consumer's perspective:
Give me physical control over the basic tasks. I always want basic navigation buttons right on the steering wheel to control navigation via screens, volume and search, etc. I want dedicated buttons for auto settings like cruise control, speed limiters that have to be set and reset frequently. I want a physical climate control that allows me to adjust the temperature and activate the heated windshield and fan settings by feeling alone, especially when the screen fogs up while driving.
Give me voice control for less frequently used tasks or tasks that require more configuration. When I have to call someone or set a destination on the navigation system, I don't want to romp around with touchscreens while I'm driving, and I prefer not to have to drive past every time.
Give me options that can be controlled with a selection wheel (like in BMW, Mercedes, etc.) or with direction buttons on the steering wheel so I don't have to try to tap a button on a screen while the car is driving around. I find this a terrible experience because it takes a lot of concentration to hit small touch targets while trying to stabilize your hand with the movement of the vehicle.
Most functions should be easy to control with both hands on the steering wheel and, if possible, with minimal focus to look at a display so that a head-up display or a display in the instrument cluster helps a lot.
I believe that touch input should only be used as a backup. If the speech recognition fails and a user has to enter their inputs manually, they can use an on-screen keyboard. If they are paused and want to navigate directly to something on the screen or quickly scroll through a list, they can use touches instead of moving the cursor with the buttons. Use touch to improve the experience and do not replace other input methods.