It can vary so much depending on size of library and if it’s a remote library the internet speeds between the source and you can change the time needed.
The best way I’ve found to get an idea of how long it takes is to as soon as you launch Infuse go to the Settings > Library and there you get the screen with the total files in the Movies, TV Shows, and Other. Just below that you will see the messages on what Infuse is doing regarding the updating. Sometimes I get a couple of syncing to iCloud messages before the “Last Updated…” message appears so if you get the “Last Updated…” message wait a minute to make sure Infuse has completed the iCloud sync. I think it does multiples when other devices have done a sync and Infuse does both checking for iCloud updates from other devices running Infuse and also it sends updates to iCloud for changes this device found.
Hope that made sense. Sometimes when trying to explain what I’ve observed I can get long winded.
Keep in mind, with Background App Refresh it can do much of this when the ATV is sleeping/not being used so that feature really helps cut the time for syncs.