How Regularly Should I Revisit My Business Plan?

This will totally depend on your business and your situations. It's essential to update your plan any time your business changes to make sure your strategies are still relevant. You should additionally revisit your plan any time you are looking for small business financing.

You should have key point indicators in the business plan and the plan should be revisited when those key points are not being met
every month would be a good point to check if the key points are being met
at this time it would be a good place to check the policies of the business to make sure that other key points ( safety levels , code of conduct, hr etc) are being met
Basically it amounts to this, the longer the period of time that you allow to pass before revisiting the business plan , the greater the risk of business failure as you will loose control of the operation