HDD failures are random. I've had drives that have worked for decades, and I've had some that went to the great bitbucket in the sky after a couple years.
My recommendation is two fold:
First, get a backup drive (or a Drobo if you have a lot of data.) Then enable Time Machine or get Retrospect X if you want more advanced features (auto mirroring of critical documents, encryption, copying of backups to other media, etc.) This way, if your drives do decide to retire and take your data with them, you have a recent backup of your work.
Second, consider another HDD and RAID. Either software RAID in OS X, or a hardware RAID card. This will definitely mitigate a single hard disk failure.
Regardless, I highly recommend enabling Time Machine and getting an external drive if at all possible.