Understanding your Core Values, how to market them, and exactly what your mission is are part of the foundation of your Company.  Think of your company as a table; one leg is your Core Values, one is your Mission Statement, one is your Business Plan and the fourth is your Vision Statement.  Without all four legs your table will tip over.
        Your Vision Statement is part of the blueprint of how you will manage your business or project.  Just as you can't build a house without a blueprint, you can't build a business without a plan. Your Vision Statement is part of that foundation.  It tells you what you are trying to achieve so you can plan the steps along the way to your success instead of acting randomly.
        The timeframe for your Vision Statement may vary from a few months for a specific project to many years if you are creating a legacy. Each project has its own timeline, its own vision and its own mission. 
         Keep in mind that your goal is a moving target.  Your Core Values should remain fairly constant, but your Mission and Vision can change as your business ebbs and flows.  The exception may be if you create a global vision; Microsoft's global vision is a good example, "a personal computer in every home running Microsoft software". Google also has a global vision, "To make the world's information universally accessible and useful". 
         Your Vision Statement is probably not as global or permanent as these examples.  More likely yours will be subject to tweeking and adjusting; you may have put limits on yourself because you weren't aware of your true capabilities, or just didn't trust yourself enough to really reach out.  In any case, editing and updating your Vision Statement keeps it fresh and gives you a very powerful tool to drive your growth and success.
        I call these principles "Power Tools".  There is nothing mysterious about them; they just give you the power to create the world of your dreams.