When I see guys here buying vintage soaps that are 50+ years old, and they still perform very well, I would say 50+ years! I think it all depends how things are stored. If you seal it well, and store it in a cool and dark place, like a basement, it should last very long.
will never break down or go bad. On the contrary, buy more soap than you need. Stock up when it's on sale or whatever. Unwrap all the soap and put it up in warm place. The older and drier and harder the soap gets, the better. Scents will mellow and change, the older a soap gets, the more you will love it.
I have 2 pucks of Yardley Black Label from the late 60's/early 70's. They both still have very potent scent and shave like any other soap I have or better. If I hadn't been told they 40 years old I never would have known.