How long does it take to upgrade from Windows 10 1909 (19H2) to Windows 10 2003 (20H1)?

I’m currently running Windows 10 Version 1909 (OS Build 18363.448).

When I upgraded from Version 1903, it was a painless process since Windows was using the Feature Toggle mechanism using a small “enablement package” (KB4517245) to switch it on. There was no dreaded hour-long install where there is a risk of having to wait another hour for roll back.

I’m currently on Slow Ring but I’m interested in installing Build 19013, the latest Fast Ring release for Windows 10 20H1. But before I commit to the update (by changing my Insider setting to Fast Ring), what is the process like? Is it using the small enablement package like 1903 to 1909, or is it like the older style “you cannot do anything while your Windows is installing a new version” style?

Also how much disk space is needed? I only have 15 GB left.