Was wondering how much smoke/petrol do you have to be inhaling every day to get lung cancer?

Large amount...: For most people. Those who smoke as well may have increased risk with combined exposure. However, lung cancer due to environmental pollutants is pretty rare ina this country. In developing nations where occassionally fossil fuels are used in poorly ventilated spaces for cooking and heat, it is more common. Smoking is still a much bigger risk.