Given sufficient airflow and good temps, the cards should last indefinitely.  GPU based distributed computing projects are known to be skilled at killing cards but usually that is due to one or more of 3 things:

1) Bad temps (bad airflow, etc)

2) "Dirty" power (low quality PSU with dodgy 12v rails are usually the cause)

3) Excessive load on VRMs.  Usually you won't have any VRM issues with cards that have enough airflow, some cards are known to have insufficient cooling on their VRMs for 24/7 100% load operation but fortunately no "repeat offenders" are common Bitcoin mining cards.  Folding@Home and GPUGrid forums will have lots of info on what cards to avoid.