A few years ago I scored two cutter-top tins of Wills Cut Golden Bar. No date but since cutter tops were replaced with vacuum-sealed tins in 1960s, they were at least 50 years old.
The first was perfectly sealed, and it was exquisite. The aroma was a sort of almond/ brandy, which hit your nose with a real headiness. It came through rich and thick on the smoke. Delicious.
The second had a rust spot. Goodness knows how long it had been open to the air. I rehydrated the contents and it was just like smoking a Samuel Gawith virginia. It had lost all of its aromatic content, but the 'plain' tobacco was still fine. Nice. Nothing special, but certainly not 'bad'.
So there are two answers to your question. The first totally depends on your tin storage, coupled with a little bit of luck. If you store it well (cool, dark and dry) and no rust appears, I don't see that it'll ever go bad and might develop into something quite exquisite.
The second answer is sort of 'forever'. Even if you do get a pinprick of rust, you should still be able to rehydrate and enjoy the contents. It might even surprise you.
Steve