Hey Mark. I teach high school Alg. 2, PreCalc, and Calculus. I would say that a lesson per day is definitely reasonable given your expectations in terms of time spent on homework outside of the classroom. I would be somewhat flexible with due dates since it will take some students longer to complete the lessons than others. I will often assign anywhere from 3 to 6 lessons per week with the expectation that they are all completed by a specific date/time (Sunday at midnight for instance).

A lesson per day is very doable, especially if you will be expecting students to work outside of the classroom on Khan.  Remember to be flexible and differentiate the lessons for your students.