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.
Who should see your plan? All the players in the company. Firms typically keep their marketing plans very, very private for one of two very different reasons: Either they're too skimpy and management would be embarrassed to have them see the light of day, or they're solid and packed with information . . . which would make them extremely valuable to the competition.