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 plan should cover one year. For small companies, this is often the best way to think about marketing. Things change, people leave, markets evolve, customers come and go. Later on we suggest creating a section of your plan that addresses the medium-term future--two to four years down the road. But the bulk of your plan should focus on the coming year.