How long gift codes "lasts" here on gog. I heard it used to last much longer before, but that they now have short expiration dates...

Gift codes you bought have never.

Some codes that GOG gives for free have expiration dates; it's usually stated in the giveaway.

You can plug any gift code into the redeem page and it will tell you its expiration date.
If it's a promo code that you got from an official giveaway, even if it expires "Never", you should probably not try to hold onto it forever, as GOG silently invalidated several such codes a year ago. (It did, indeed, used to be the norm for promo codes to be valid indefinitely, but I imagine publishers don't like that, quite understandably.)

And like ZFR said, if you paid money/wallet credits for it, it will never expire, even if the game gets pulled from the store before it's redeemed.

You guys are nice with fast responses. :) Thank you.