Showing reading time or a progress indicator on articles is a relatively new trend, now you can do that for your next project with this snippet. There are 2 variation one with JS and one in pure CSS. The initial version was designed by Derek Palladino and the pure CSS version was done by Mike Riethmuller. When you scroll down the page you can see a progress bar just below the sticky navigation bar section.
If you are looking for a scroll indicator in the sidebar check out this snippet.