By the "how long can wort sit" question, I'm guessing you've already brewed.
If haven't brewed, make a starter and see if you get any action.
How long ago did you smack the pack? It could still be viable yeast, just slow to start.
From Wyeast's site-
5. How long should it take for a package to swell?
If a package is fresh (within 2 months of the manufacture date) the package should show signs of swelling within 5 hours and typically much faster than that. When the yeast is stored for long periods of time, they slowly consume their energy reserves (glycogen). When the energy reserves get low, the yeast are slow to produce CO2 and therefore are slow to cause swelling in the package. Improper storage at warm temperatures also has the same effect as long storage times
You're not going to hurt anything by pitching the yeast as is. I'm not sure how long wort should site before yeast is pitched, but it's usually as little time as possible. If you have a homebrew store nearby and want to have some piece of mind you could just get some fresh yeast and pitch.