I guess it depends on how many files you have and their size, plus your connection speed. But it has to download every file on a new device and that can take some time.
I've noticed this happens in one of two scenarios after a fresh install:
If I haven't installed Pages, Numbers, and Keynote, none of their files will appear right away. Once they're installed, the files in iCloud Drive will show.
If there are no files in an app's particular folder on iCloud Drive (i.e., Scanner Pro, PDF Expert, etc.), it will not show until I place a file in that folder, or until I reboot.