How long do messages take to download from iCloud after initiating sync?

I was updating my iPhone SE (3rd Gen) to latest IOS today using my PC when it errored out mid-update. I initiated a backup to find that my most recent was in the middle of September. I had iCloud messages toggled on in settings, but they have fully "synced" a couple times but my messages between September and now (October) aren't coming back. Are these lost forever or just taking a long time to download in the background?