The creator of the Purpose Planner has always worked in corporate environments. 6 months is a visible time frame, much like 2 quarters. The plans you commit to in that time frame have to be tangible and realistic for you to take action against them. Typically, most notebooks last around 6 months as well so this was also a consideration. Also, at 200pages, it is just the right size to carry around without becoming too bulky and heavy.