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.
Top-level reflection: In the daily hurly-burly of competitive business, it's hard to turn your attention to the big picture, especially those parts that aren't directly related to the daily operations. You need to take time periodically to really think about your business--whether it's providing you and your employees with what you want, whether there aren't some innovative wrinkles you can add, whether you're getting all you can out of your products, your sales staff and your markets. Writing your marketing plan is the best time to do this high-level thinking. Some companies send their top marketing people away to a retreat. Others go to the home of a principal. Some do marketing plan development at a local motel, away from phones and fax machines, so they can devote themselves solely to thinking hard and drawing the most accurate sketches they can of the immediate future of the business.