Actually it can be a little bit tricky to get it right. I found a good German description how to set up your own custom column that shows the reading time:
https://www.e-reader-forum.de/e-book...lten/?pageNo=2
Even if you don't speak German you should be able to follow the code and the screenshots.
The real fun started when I wanted to convert the floating point number into a string with hour and minute (e.g. 6.7 -> 6:42h). The template functions are very limited so I came up with some ugly code to do the job.