How long can you leave your diesel car without starting?

I can put it this way...diesel will 'go bad' in less time than gasoline, in a year it will have lost 20% of the energy contained in a unit. But to really answer you, it can be stored for many years if "mothballed' properly. Just parking it, don't expect it to start in 6 months. Condensation will form in tank, battery will frow weak...and it might start but after say a year with no prep, unlikely