Not as long as you might expect. Average lifespan of cars in Europe is 18 years. Sure there are outliers and cars might have a longer life in poorer countries but that is a good starting point. The fossil fuel ban on new cars is set to take effect in 2035. So even if a car is bought on 31.12.2034 it will likely fail by 2053. Just 10% (on average) of cars fall under this category. The other 90% will probaply fail well before that and be replaced with a vehicle that's not using fossil fuels.