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.

I'm a firm believer that a business running on strategy, implemented well, can't fail. The reason is that good implementation includes monitoring progress and adapting when needed. Even the best strategies don't always pan out due to changes in the market, competitor responses, execution challenges, etc. But if you're monitoring, measuring, and adjusting your strategies along the way, you're able to navigate the challenges and ultimately succeed.