For the past 4 years, it has been 10-<30 minutes. 1 of the upload in March 2022 took ~15 hours to finish. For those who are not aware, use the Appstore Connect App from iOS Appstore, it will give notifications once the processing is finished and if the apple review finishes.

I hit this issue 2 days ago and was lucky today. It is recommended to check the browser console on the submit button click to see the server response to see if you can get a more descriptive error. I got a generic error hence it didn't help. Things to try out, as recommended by Apple support. Try Safari this time if you were trying from other browsers. In Safari, App Store Connect > Make sure to remove the old build > click save > refresh page > Select the latest build > click add to review. If that didn't work retry step 2 from a different manager account this time. If you are still facing an issue take a full-screen video record of this and give it to them at https://feedbackassistant.apple.com/ They will respond in their business days. As I tried today (29 Jan 2022) the app store connect website looked different than how it looked one week ago. and today I was able to submit it successfully from the Safari browser. I'm guessing Apple did some upgrades and servers were not fully ready when I tried (multiple times) past 24 hours