Soap scum noticeably builds up on shower doors every two weeks. There are several ways this can be eliminated. One is using Sprayway Glass Cleaner and wiping it away. If it has tons of layers for months and months, it can most effectively be removed with spraying CLR on the glass & scraping the buildup off with a paint scraper. Be sure to use a new blade each time. I've only found two shower doors that I couldn't make look new again. And they drove me crazy!
Here's a shower door in progress:
(Notice the left side is noticeably better than the right side. So, you can see what a huge difference a little elbow grease makes on the glass. Because, obviously, both sides looked like that this morning.)