It varies greatly between HDDs
There's a value called MTBF specified by HDD makers
MTBF is an abbreviation for Mean Time Between Failures.
MTBF is a measure of how reliable a product is. MTBF is usually given in units of hours; the higher the MTBF, the more reliable the product is.
For electronic products, it is commonly assumed that during the useful operating life period the parts have constant failure rates, and part failure rates follow an exponential law of distribution.
I'd put down the formula for the probability HDD would fail after T hours:
Probability of failure = exp(-T/MTBF)
Thus, for a product with an MTBF of 250,000 hours, and an operating time of interest of 5 years (43,800 hours):
Probability of failure (R) = exp(-43800/250000) = 0.839289
which says that there is an 83.9% probability that the product will operate for the 5 years without a failure, or that 83.9% of the units in the field will still be working at the 5 year point.
Average failure rate of the HDD is when the drive has 50% chance of failing
So if you have 2 drives, you expect 1 to fail while 1 survives.
for R = 50%, T = -MTBF*ln(0.5)
For example, Western Digital Raptors have a MTBF of 1200000 hours
On average after 866433 hours it would fail, which is 98 years
However it doesn't mean it won't fail before that
4% probability that a Raptor would fail within 5 years