Prizes don't expire...
that's the clear response I got from CS when I asked.
As I did not keep the e-mail, I can't post it, but they told me in no uncertain terms that I could accumulate cash as well as the smokers, watches, basketballs, necklaces, etc. as long as I want to and claim at a time of my convenience.
In short, as I understand it, a prize can remain unclaimed for an indefinite period, but when you're ready to claim you need to produce the identification required.