With proper care the card should last long enough for you to not want to use them for mining any more. Maybe only few faulty fans.
Proper care in my mind means:
- keeping the temp in check with moderate overclocking and undervolting
- keeping the cards in dry place and relatively cool place
- cleaning the dust from the fans every few months
+ if you like DIY, you could change fans from small high-speed to bigger low-speed and save original fans for selling cards 'like-new'