Do you sit in the car with the engine running and air conditioner on? In a newer car with a modern cooling system you could run the AC for days at an idle without doing any obvious harm. In an older car with a weak/poor engine cooling system, the car can overheat. The best advice is to try to keep the car in the shade and keep an eye on the temperature gauge. Whatever vehicle you have, it’s best to leave the windows down a little. Never sit in an idling vehicle without ventilation.
Unnecessary idling is, however, costing you more in petrol and environmental impacts, and adding wear/miles to your engine. Whenever the engine is running, you are slowly wearing out the electrical components, belts, etc of the vehicle without racking up any ‘highway miles’. Doing this for a few minutes here or there doesn’t matter but 30-40 minutes at a time is generally considered a bad idea.
While idling was once recommended to warm up your engine before driving, this is no longer necessary or recommended. Once the engine idle speed has stabilised, which should occur within a few seconds of starting, you can drive the car. A cold engine uses more fuel than a warm engine, and the engine will warm up faster while driving rather than idling.
Another commonly held belief is that diesel engines need to idle for 5 minutes or more when first started. In fact, as gelling of diesel fuel is no longer an issue, most engine manufacturers recommend that diesel engines run for no more than 3 minutes before driving. Diesel engine idling causes twice the wear on internal parts compared to driving at regular highway speeds, which can increase maintenance costs and shorten the life of the engine.