This is not easy to know at first: you usually get better at this as your experience increases. The best idea is to over-plan so that you don't run out of instructional activities. If you have prepared extra information, you will be ready if your time runs longer than expected. Organize your lesson plan so that the most important parts of the lesson -- the areas that absolutely must be covered-- come near the beginning. That way you won't find yourself running out of time and not getting to the main part of your instruction. Also, in the beginning it might take some adjustment to monitor yourself and consider where you are in the classroom period to know how to adjust your lesson plan. Be patient, this is a skill that comes with time and practice. Before long you'll find yourself knowing just how to pace a lesson and what to add and what to remove. There are two basic ways to think about how much to include: How much time do you have? Consider the amount of time, and the objective of the lesson, and then design the lesson around this, thinking of each segment of the plan and how long it will take to implement. What do you need to teach? With this approach, consider your objective first, then think of how much time it will actually take to accomplish that. Will it require more than one lesson period? How can it be divided up? What can you do in a single period and how will you continue the lesson? Do you need to design multiple lessons? When you start from one of these approaches, you will find yourself able to consider the necessary amount of information to include in each lesson.