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
also you need to check the conections between gpu and psu and motherboard in order to see if all is OK or there is something wrong