statistics - **How** harmful is a hard disk spin cycle?

It is conventional wisdom¹ that each time you spin a hard disk down and back up, you shave some time off its life expectancy.

The topic has been discussed before:

Common explanations for why spindowns and spinups are harmful are that they induce more stress on the mechanical parts than ordinary running, and that they cause heat variations that are harmful to the device mechanics.

Is there any data showing quantitatively how bad a spin cycle is? That is, how much life expectancy does a spin cycle cost? Or, more practically, if I know that I'm not going to need a disk for X seconds, how large should X be to warrant spinning down?

¹ _{But conventional wisdom has been wrong before; for example, it is commonly held that hard disks should be kept as cool as possible, but the one published study on the topic shows that cooler drives actually fail more. This study is no help here since all the disks surveyed were powered on 24/7.}