How Often Should You Review Your Marketing Strategy?

The two main lifestreams of any business are marketing and sales; if you have no plans for marketing, you do not have a plan for sales- and you, essentially, are building a business that will not be able to survive. So if you have no marketing plan, or it is not written down- begin by first creating one and writing it down. However, once this is in place, your next question might be, How often should I review or change my marketing plan?