As a first test to check if everything is just going fine but really slow, try this:
- Keep the App Store window open (thanks to @Dhruv Saraswat to point this out).
- Open the “Console” app (not the “Terminal”, but the “Console”)
- Go to the “Search bar” and type “App Store”.
- Push “Start” button.
- Log lines will be added from time to time showing you how the installation process goes. In my case, I saw “… Completed: 825 of 1000”, and some time later I got “… Completed: 826 to 1000”, and so on.
That way you can at least check if everything is working, although really slow. You can then guess how long it will take to the “Completed: 1000 to 1000” step in your own situation.