Longest I've gone without dumping is 21 days.
If you're wondering what people through a series of unfortunate events had not been able to dump before the RV was stored, I've read of some stories that would curl your hair. One owner I recall decided to remove the tank and dispose of the entire tank rather than try to liquefy the dried out mass inside. The drying out can happen to either tank, though the black tank might be more bothersome.
The good news is that depending on conditions, ambient temps, humidity, etc., that the tank(s) were subjected to while in storage...and for how long...it's possible to restore them. If you can fill the tanks with water and add one of those enzymatic products to digest things, it's possible to clean out a tank that's gotten a bit dried out.
Might take several treatments, and lots of water, but it can restore the tank or tanks to useful operation.