In broad terms, once your drive has got through the first few months of use you can reasonably expect it to last for at least another four years with average use.
I have external drives that are five years old and still going strong, but they are not used every day. All of my critical data files are backed up to the Cloud (Microsoft's OneDrive) every day.
You will usually (but not always) get some warning when a drive is going to fail - it may become noisy, or start giving trouble when you run searches - that kind of thing.
If you have really important files, you can't do better than running a backup routine to a reputable Cloud service. Your data will be on secure servers which are mirrored at frequent intervals - it's about the safest storage method you are likely to find, and of course you can access your files from any machine, anywhere in the world.