The amount of time you spend doing activities in your lessons is recorded on your Performance page. The Spotlight box at the top of the page shows how many activities you've completed and how much time you spent answering questions in those activities. The Spotlight does not calculate the time spent reading articles or listening to conversations and recordings. You should read and listen at your own pace. Take as much or as little time as you need doing this, it won't affect your study time calculation.
Make sure to check your Spotlight regularly; it resets every 90 days so you get a fresh count of the activities you have completed.