For reference - and IMO this should be taken with a grain of salt, mean time between failures (MTBF) should be identified either on your hard drive or in the documentation for your hard drive. Some manufacturers are now using a new calculation, Annual Failure Rate (AFR).
MTBF is defined as "the probable average number of service hours between failures."1
AFR is defined as "the probable percent of failures per year, based on the manufacturer's total number of installed units of similar type."2
There are many different factors that go into determining how likely your drive will fail. Everything from manufacturer's defects to how many read write operations your perform to the climate conditions where you store the drive impact the actual lifespan of your drive.
Ars Technica has a good article about AFR here.
PC World has an article about a study that was conducted which found that hard drive failures are more frequent than estimated by manufacturers here.
So Saw is right in that it is unpredictable, but there are statistics that may give you an idea of how long your drive should last. You need to keep in mind though that is an estimate and many factors may result in a different real world answer concerning how long yours will function.