How long can a car engine sit for?

I have known of engines to sit for 50 years and start right up

Running? Until it runs out of fuel or something breaks.

Not running and either in or out of a vehicle? Depends where and how it's stored. The better conditions it's stored in the longer it will last. If it's left outside it will eventually rust and be no good whether it's covered or in a box or in a vehicle or not.

There is no *this* amount of time answer for your question. It depends how it's stored.

Mine rolls over and plays dead