Hard Drives on web servers typically last from 5 to 8 years but sometimes they're replaced before they actually fail just to prevent any data loss.
Most data centers will go through there web servers and when they notice that the hard drive is nearly at the end of it's lifespan or they start noticing any faults with the hard drive then they'll replace the hard drive in the web server because the data that is stored on web servers is most always important enough that you don't want to lose the data should the server crash.
It's still a good idea especially if you run a website and host it on a web host server you should back up your data locally or to another online data storage backup website to keep your website database and files protected.