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.
Ideally, after writing marketing plans for a few years, you can sit back and review a series of them, year after year, and check the progress of your company. Of course, sometimes this is hard to make time for (;there is that annoying real world to deal with);, but it can provide an unparalleled objective view of what you've been doing with your business life over a number of years.