Using word count and average reading rates could create an unreasonable benchmark which can be frustrating to learners. Reading speed varies greatly depending upon the level of difficulty of the content as well as the vocabulary level, familiarity with content, and reading proficiency learners bring to a course. Also using word counts does not account for other learning activities (required or undertaken independently).  

I'd be inclined to post estimates (and label them as such)  for a new course and then ask the students for feedback about how much time they spent. You could ask them to log their tine spent on each block or overall and make it a low stakes activity. You may also be able to gather this information through your LMS. Then you could give subsequent groups a range of times based on actual learner experience, and you'll know whether your estimates were realistic  You could even do a cross-comparison between time spent and marks achieved. It might prove an interesting starting point for discussion.