Neither activity is timed when completing a Learn or Practice exercise. The required elements, tasks, in each Lesson exercise varies in regards to effort or time to complete.
The most significant difference is that the Learn exercise can only be completed once and as such, should be completed in one sitting. However, if needed, there is an exercise "Save" feature. The Practice exercise can be taken multiple times and is there to help all students further reinforce their knowledge in that particular skill set.
Remember, at any time you can click the eBook button to read more information about the feature or to strengthen your knowledge as you progress through the exercise.