It depends on size, we have over 60,000 computers and before End of Support for XP we moved to 7 in under a year. That was a kick in the butt and Win10 has been far quicker. It's the standard and almost everything deployed gets it, but there are always stranglers who are slower. You have to take from other projects to do a mass upgrade and it can be a hard sell when it will happen on the systems renewal schedule anyways.
And well, in this case, corporations aren't just slow to switch, they're also still investigating, if they can actually even use Windows 10 due to the increased telemetry.
For many companies, deploying Windows 10 would mean breaking contract requirements or the law.
Just two weeks ago, the ministry for data privacy of Bavaria released a report, that hey, they can actually legally use Windows 10 Enterprise, given a list of tweaks that have to be made in each deployment.
And then the addendum was that new privacy laws are going to become active next year, so at that point, these results are completely irrelevant, again. I expect the next report on Windows 10 being usable (or not) in two years from now.
Despite saying IOT in the name it's really intended for POS systems.