The most honest answer to this question is that you should be updating your business plan constantly. Whenever something changes, you should update your plan. By updating it consistently, it will become a long-term planning process that sets up your strategy, objectives, and the steps you need to take.
It might not be realistic to update your business plan constantly. So, you should aim to update it at least once a year. An essential part of the planning process is watching the difference between the plan and the actual results. You need to be keeping track of where, why, and how you were wrong. Tracking your plan in this way is key to effective management.