It's a difficult question to answer. I suggest you watch Gamers Nexus latest Q&A video on youtube, he addresses this point specifically. The general understanding is, as long as your card is not temp throttling, you should be fine. What's most likely to pop are capacitors. They are rated between 5000 and 10000 hours usually.
That's like 200-400 days? Scary... Even though there are plenty of ~5 year old HD79xx/R9 28x, that are still mining now.
amit_talkinOut of these two I'd prefer nicehash. Profit is higher there.