1 Million hosts over 45 days = 15 min per host

That's a very realistic/optimistic number (especially as you do want to wait for all services to be running and marked as healthy)

"Oh but you can batch this" sure, but you don't want too much of a big batch that will make your service slow or want to risk shooting yourself in the foot - like rebooting your whole control plane then figuring out it doesn't work like that

(The 45 days is probably an estimate as well, I'm not sure they actually do that server by server)