The notebook is designed to be used for 6 months. In all of our tests, we found that people tended to fill up notebooks in that amount of time. We thought about doing 12 months of weekly layouts, but then it would just be a planner and have no room for notes.
Of course, all of the layouts are undated, so if you miss a week (or a month!) while you are on vacation, you can still keep using it! One of our goals is to make every single page useful, no matter when or how you use the notebook.