Installers measure completion by the number of files installed and tasks completed, so there are usually patterns to where they get stuck.
Office tends to hit one snag at 90% for some users and it generally means that Windows Installer is installing other updates that Office needs to wait for.
If you haven’t updated Windows in awhile, you could have a big backlog, so what’s happening is that Office reaches 90%, then essentially pauses, and Windows Installer takes over and installs each update in the queue. Unfortunately, this is not shown on the Office installer, so you are left with the sense things are just frozen.
We recommend running the installation overnight so that you don’t have to sit around waiting. If you wake up and it’s still hung at 90%, it’s time to call Microsoft support for more personalized help.