No. All the lessons vary is length and content, and there is no time restriction on completing individual lessons. You do not treat these lessons like you would an hour’s lecture in a classroom, for example, you can do as much or as little as you like when you have the time free to do so.
NOTE: Your progress bar (on your contents page) will only take into account the lessons you have completed (either by clicking the ‘Complete Lesson’ button at the bottom if the page, or by reaching the pass mark required of the relevant quiz).