Do guest passes have an expiration date?

nope

If it does it usually says so on the guest pass itself. If it doesn't say, then probably no.

Some may, but I think KF guest passes do because I had one when I bought the game and it's no longer there. I never used it or gave it to a friend.

My Shattered Horizon and Killing Floor guest passes are gone so I guess so!

Yep. Pretty sure they all do. My CSS, KF, and Shaterred horizons all expired.

I had some killing floor and red orchestra guest passes, and they eventually disappeared, maybe 1 or 2 month after I got them.

I think the Killing Floor ones do not have an expiration date

Yes, but they don't tell you when they expire.