When does Messages in iCloud actually sync? I just updated my iPhone 6s to the latest iOS version and turned on Messages in iCloud (Settings → my name → iCloud → turn on Messages).

However I don’t think anything got uploaded to iCloud, per the below screenshots showing my iCloud storage. When/how does this happen?

The first photo shows my iCloud storage breakdown. It seems to suggest space being used for messages, but after tapping into the messages detail you can see Zero KB are used for conversations.

Also this was immediately after enabling Messages in iCloud so I’m skeptical that it could have uploaded 600 MB that quickly (and there was no network activity indicator in the upper left hand corner of my iPhone) But about 15 minutes later it hasn’t changed.

Does it just happen when iPhone isn’t in use? Overnight? How will I know when it is done?
This happens with the same criteria as iCloud Backup: when the device is plugged in, idle (overnight) and connected to a Wi-Fi network.