How much do spin-ups wear out a hard disk drive?

How much do spin-ups wear out a hard disk drive?

On a mechanical hard disk drive, roughly (orders of magnitude) how many
hours of continuous operation is one spin-up equivalent to in terms of
abrasive wear?
The reason I'm asking this question is because I have an SSD and a HDD and
I want to decide whether I just want to make the HDD be continuously on
(if that setting is even possible).
EDIT: Ignore the following question (as etienne points out the flaw in the
comments)
Further, when you hear the HDD spin up when the PC is on, will it also
place the R/W-head on disk (like it does when you start up the computer)
or does it just spin-up with the head already on the platters? I'm asking
because the degree of abrasive wear could be higher in the case where the
head is placed on the platters as well.