As long as the cooling system is working properly, you should be able to sit in any modern car you buy and let it idle indefinitely. Or at least until you run out of gas. Or need to pee.
In a modern car properly designed with sufficient cooling system, almost indefinitely. The engine should be able to idle and power the air conditioning and not overheat, even on a hot day in a tropical climate. Eventually you will run out of fuel and that will be the limit.