Thanks for sharing, Nicole. At SweetRush, we tend to use 150 words a minute to determine the seat time for content within a course.
This number aligns exactly with the audio in courses. Though many of us tend to read faster than this on text-based pages (those without audio), using one number makes things easier and adds that bit of time the learner uses to look at infographics or re-read/digest more complex content. In addition, practice activities and knowledge check pages can take anywhere from .5 minutes to 2 minutes each depending on their design and complexity.
After several pilot tests with our clients, this method has proven to be pretty accurate when you look at the average course completion time (as you mention).