how often should I update my business plan?

The short answer is any time you feel it no longer accurately describes your business activities or outlook. That said, minor changes to your org chart or a new competitor in town probably doesn't warrant an update. Focus your updates on more significant changes in the market, your business operations, or plans. Is the market being impacted by new regulations? Are you taking on new ownership or rolling out a new product line? Are you making a major new investment? Are your sales substantially off from projections (20% or more)? Finally, even without major changes, you should revisit your business plan at least once a year to refresh the market analysis, business descriptions, and financial projections. Doing so around the time of your fiscal year end is best practice.