Found one in 2006 that was in the hands of an old-timer (who didn't log most of his finds online) for over 2 years. It was still logged into the cache he took it from in 2004.
I know someone who's been holding on to a TB (not mine) on my watchlist since 2004. This guy is local to me and is alive and well. I expect it to end up in a cache someday.
I believe the report of the longest missing TB resurfacing that I've seen in the TB forum is about 5 years. Hopefully this thread will not get moved over there, or you'll be cursed to only getting a handful of replies to the thread.
Majormajor is correct. It was the fall of 2001 when TB's became available to the general public, and went into widespread circulation. So they've been around about 8 1/2 years.
That gives us hope that our missing items might be someday reapear again.