It is almost impossible to predict when a hard drive will go bad. I've had desktop hard drives that went bad after 9 months. I've also had some that are still working after 10 years (and I still have a Tandy even with a working hard drive!)
Sometimes when a hard drive begins to fail it will perform very poorly and/or exhibit the "click of death." When either of these things happen(especially the click) it is time to buy another one and stop using the hard drive until you are ready to back it up. Note, the few hard drives I've had that failed, failed suddenly without any slow downs. They just one day didn't work.
Other than that, to ensure a long hard drive life is not to bang it or other rough hard movements. Also, store it somewhere between 70-100 degrees Fahrenheit and run it in such a temperature range. Also, make sure it isn't in an overly dusty environment.