The basic principle behind backing up a system routinely...is that none of us knows what the future holds, except in hindsight.
A comptuter is a highly complicated tool or toy (depends on user). It has moving parts (hard drive) and electronic parts which most of us do not understand the workings of. Even if we think that we understand...the fact that any part may crap out at any time (do a Google sometime on the shipped "new" components that arrive at purchasers DOA) makes a computer a device that is a problem waiting to happen, IMO...just as your next step outside your door is an accident waiting to happen.
If that doesn't explain the need to conduct routine backups...then I really don't know how to get that message across.
Since the hard drive is, IMO, the computer component most likely to fail or degrade...and since Windows and all files are stored on hard drives...backing up the drive contents is the only way to mitigate the risk users face daily...anytime they do anything...involving a computer.
I consider backing up...essential preventive maintenance...for anyone using...troubleshooting...or attempting to otherwise "repair" a computer situation where the hard drive is involved.
I cannot estimate how long a diagnostic will take...depends on size of drive and problems encountered, I imagine. I know that it should take longer than it takes a chkdsk /r to complete on that drive...beyond that, I have no clue. I don't time such things...the time it takes is not important to me.