Difficult question. What do you really mean by ‘how long my e-learning will take in total’? There is a big difference between the ‘editor time’ and the time spent by a learner in your course since you probably will have interactions. Th editor time is the sum of the durations of all the slides and easy to detect. You can see it by default in the Table of Contents or in the Project Info panel (see attached screenshot).

Guessing how much time your learner will spend to finish your course is a lot more difficult and can only be tracked on runtime. I published a blog post to explain the workflow:

https://blog.lilybiri.com/display-time-cp2019