how long does it take icloud photo library to prepare on pc?

I have downloaded icloud for PC and would like to download pictures and videos from icloud to my PC. In icloud photos, when I select "download photos and videos", I keep getting a message indicating: "you photo and video library is being prepared...it may take a while".

How long will this take? I have tried all day, and keep getting the message and see no progress/status.

Thank you!