Hey shengliu,
Quite a while. I live in Australia and the humidity is pretty high most of the year.
I have cards in my collection that I’ve had for more than 15 years that are still brand new.
The humidity will start to effect the deck badly once they have been opened and used extensively.
I have some decks that are more than 40 years old and it’s clear that somewhere in that window the humidity has taken a toll on the cards as they simply don’t perform well despite being big brands of the era.
But, I haven’t faced any of these issues with modern cards made in the last 15 years.
Maybe at some stage they will be affected but not yet.
Hope this helps.
B.