You took EarthDog's figure assumption as correct, but I question it because I install every Windows 10 build on my personal systems and have found out the hard way that I need way more free space than that to install every iteration of Windows 10 as it comes out. I have plenty of space but it is on other partitions, since every single HD I own is partitioned, I have no need for a large OS partition, in fact I keep it as small as possible for drive imaging speed purposes. I still had to increase its size as time went on because Windows 10 will not install, even though I have terabytes of free date on the system, Windows 10 will only install if much more than what EarthdDog assumed is available on the OS drive itself, even if you have terabytes of free space on the entire system.
Second, equally important, I have *never* kept a single personal file on C drive after losing some personal files in the mid 1990s due to a Windows glitch.
So I keep the OS partition just big enough to be operational with no actual programs installed on it [they are all installed on different partitions so that Windows can't get to them - for real this was the reason, which now is no longer paranoia but a real danger] - so I advise everyone to *never* keep any files on C: Drive. I don't even install programs on C: drive, let alone keep personal files there.
I also don't know what the figure is, but let's not forget before blaming the victims, that Microsoft made updates mandatory, not optional on Windows 10.