Even the best Marketing Plans become outdated. In business, change is relentless. Your competition reacts to your successful offers, new players enter your market space, technology provides new opportunities, external factors offer new threats, and the list goes on.
Your marketing plan should provide short-term protection in the form of ongoing evaluation of both quantitative and qualitative factors as well as ROI. However, a review of the plan itself should occur on a regular basis.
How often this audit is performed depends on many factors, but most companies should perform at least a cursory review annually. Marketing audits of solid marketing plans are usually inexpensive and painless. It's like the old oil filter commercial — pay a little now or a lot later!
Finally, regular marketing plan audits almost always produce the collateral benefit of refocusing the organization's attention on their marketing efforts.