Is there a way to view the duration of each slide in a .story file or add them up to get a total duration?

Two ideas,

First, lock the slides so they must be on the slide for "X" until they can move forward. Lock the next button. 

Second, most LMS systems will record how long a learner is in a class. This only answer the question of how long they had the file open. 

We don't require our learners to stay in a class for a set period of time. But we do require the learner to view all information on each slide. We lock and restrict the time slide and the next button until the timeline finishes, or they have completed all interaction on a slide.  

Hope this helps