How long will the sticky adhesive last on post it notes?

You may get the best answer if you contact 3M directly and ask them if they can confirm for you- from personal experience, I was handed down a few packages of post-it notes from my aunt, who bought them in the 1980's but never used them- it's now 2011 and they're still sticking OK! Not as strongly as new post-its, but they could have just changed the adhesive formula since the '80s. My guess is it does last a long time.