It's not suppose to. Acoustical caulk is supposed to stay pliable. Exactly how long ... don't know. A quick google came up with this:
Quote
Acoustical sealant has the capability to permanently retain its flexibility, allowing it to work effectively for a very long time. You can think of it as a non-hardening caulk.
I imaging you need to check with the manufacture of the product you're using to get a real number.