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.
Firms that are successful in marketing invariably start with a marketing plan. Large companies have plans with hundreds of pages; small companies can get by with a half-dozen sheets. Put your marketing plan in a three-ring binder. Refer to it at least quarterly, but better yet monthly. Leave a tab for putting in monthly reports on sales/manufacturing; this will allow you to track performance as you follow the plan.