There are some utilities which will tell you what your HDD condition is - you need something that will read your SMART codes. I have some 250GB drives in our business computer here - one has 45,000 hours, the other has 102,000 hours. That comes out to 5.2 years for one, and 11.7 years for the other. Both have zero pending sector counts and zero reallocated sector counts. Those really are the numbers you want to look at. Hours, how many times powered on/off - those have little effect (well, except for maybe hours) to how reliable a drive is.
Pending sectors means the drive couldn't read a sector, and is going to try later and move the data off it to it's spare area. High counts there are not good as it means you have data which might be lost and/or unrecoverable. If it succeeds in moving it, it will move that number back down.
Reallocated sectors means you had failures and sectors were remapped to spare area. This number if it starts going up indicates failing media. Once it goes up you'll want to start looking into getting a replacement and cloning the data over.
Of course, none of this takes into account a sudden failure (internal breakage) or electronics failure (controller failure) which can happen at the drop of a hat. It is possible to recover from controller failure (buy a controller board from an identical hdd and replace it, then get the data off asap). Internal breakage though - there's not much to be done about that beyond having a good backup policy in place.
For my non-critical data (ie: OS and programs) - I occasionally backup the OS. For critical data:
Data is backed up to a secondary HDD or SSD internally daily.
Weekly - is incrementally backed up to an external drive.
Monthly - is backed up and copied OFFSITE to my storage system.