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.
The results of marketing planning focus on the consumer. This can lead to competitive advantage through lower prices than competitors for the same benefits or uniqueness of benefits that can cover a high price.