I agree with Ed.
It can depend on how hard it was hammered in its lifetime, and whether it was regularly defragmented/otherwise maintained.
Then again, some drives can take a real hammering and survive for years - I have a WD Caviar 420Mb drive that I bought 8 years ago that still works perfectly. I also have a WD Caviar 1.2Gb drive I bought 2 years later, that got less usage than the previous one, and hasn't worked properly since 1998. Should chuck it out really.
Of course, mechanical devices are more prone to failure than solid-state ones, but even solid-state devices can pack in unexpectedly. Case in point, one Intel pentium III 450Mhz that died 1 year and 5 days after I bought it.
Erm, back On-Topic, you should reasonably expect a hard disk to last about three years. I don't think any manufacturer provides warranties longer than this.
With good maintenance, it should last even longer

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