How often should a marketing plan be revisited?

As often as needed...(old answer but still true)

How often you come back to your "marketing plan" should really depend on your business plan. There are many different types of business plans, some people actually confuse their marketing plan with the business plan and you should understand the difference to start with. Your business plan should be an overall strategy of how your business should be ran and how that will translate into revenue and how you plan on achieving this. Of course part of this overall strategy should include marketing, but this is only one part of the document.

When you are setting up your business plan you should set targets, goals and other metrics that should be measured and workers or employees should be made accountable to. These are the guidelines that should be used to tell how often you need to go back to the drawing board on your marketing plan or at least when a tweak is needed.

Checking your effort compared to the return you are seeing on lead captures, sales or whatever other metric is important to your business should be measured at least weekly to assure that you are on track an not just spinning your wheels.