You should allow yourself a couple of months to write the plan, even if it's only a few pages long. Developing the plan is the "heavy lifting" of marketing. While executing the plan has its challenges, deciding what to do and how to do it is marketing's greatest challenge. Most marketing plans kick off with the first of the year or with the opening of your fiscal year if it's different.
What's the relationship between your marketing plan and your business plan or vision statement? Your business plan spells out what your business is about--what you do and don't do, and what your ultimate goals are. It encompasses more than marketing; it can include discussions of locations, staffing, financing, strategic alliances and so on. It includes "the vision thing," the resounding words that spell out the glorious purpose of your company in stirring language. Your business plan is the U.S. Constitution of your business: If you want to do something that's outside the business plan, you need to either change your mind or change the plan. Your company's business plan provides the environment in which your marketing plan must flourish. The two documents must be consistent.