As a general rule I replace them every 18-24 months. With the price of a hard drive these days, why wait for it to fail. Remove the old one with your music library on it and put it in a safe place. Should all hell break loose, you at least have a backup with a bulk of your database on it.
Of course, unless your studios burn down you shouldn't ever need the "in case of emergency, break glass and use hard drive" option. You can have RAID running, an external HD to back everything up to and even a PC at the transmitter site as an off site backup with everything ready to roll on it. There's also options such as Carbonite which aren't very expensive.
Hard drives are cheap. The labor to reload your entire library isn't. Don't wait for them to fail, and have multiple layers of backup. There's no reason not to when you can get 1TB of storage for under $100. Even the cheapest station owner will understand the consequences of not spending the $100 if you 'splain how long it will take to reload everything and how much revenue will be lost from missed spots.