Every time I toggle off I get the same message "Messages in icloud messages have not been fully downloaded to this phone. Disable and Download Messages?" I click yes but it never seems to download anything. This message pops up every time I toggle off. This last time I toggled off and back on it did seem to add a couple more conversations to iCloud but it's still only a fraction of what I have and doesn't seem to be adding any more.
If you are looking at 'conversations' in settings > appleid > iCloud > manage storage > messages that is just showing conversation that include data outside of the chat database. A message that is only text based will not show up there.
If you want to see how fast messages sync find on of those conversations that hasn't been updated "today". Click on it, send a message to that person(s) then go back to iCloud settings. When it says "today" that is how long it took.
For me, on wifi its near instant.
Do you have multiple devices that support Messages in iCloud? If so you can delete a chat from one and when it deletes from the other devices is how long it takes to sync as well. If you don't then you may want to just turn it off because turning it on removes messages from your iCloud back ups since they are stored on their own similar to iCloud Photo Library.