Do guest passes have an expiration date?

Solution 1:

In games > Gift and Guest passes you will see an "expiration" column. So yes I guess it can expires.

https://support.steampowered.com/kb_article.php?p_faqid=549#expire says :

Do gifts expire if they are not redeemed in a certain amount of time?

Gifts will not expire once they have been issued. Once the gift purchase has been activated, the game is permanently subscribed to the Steam account that it was redeemed on.

Solution 2:

Yes, there are expirations for Guest Passes.

I don't know where this expiration date is listed for the person receiving the guest pass, but for the person giving you the guest pass, the expiration date is listed on the Gifts & Guest Passes interface (Games -> Manage Gifts and Guest Passes in the Steam client's menus.)

From the Guest Passes Guide:

Guest passes have an expiration date that is determined by the time they are granted to you. To check when your guest passes expire, go to Steam's Games > Manage gifts & guest passes... menu. If you do not give out your guest passes during the determined period they will expire.