Strategic plans are maps of where businesses want to go in the relatively long term. These plans must constantly be subject to reassessment when circumstances require. They should particularly be revisited and perhaps revised when there are important changes to the business environment.
If we think of the metaphor of the strategic plan as a map, we can see that there are times when the plan must be reevaluated. If you have a map that shows you where you are trying to go and you find out that a bridge on your route is out or that there is terrible weather at some point, you will need to think about finding different ways to go. The same is true of a business. Let us imagine that your firm has set out its strategic plan, only to have a major new competitor enter the scene. Alternatively, let us imagine that a new government regulation has an important impact on your industry. In cases like this, your firm will clearly need to revisit its plan. It will need to think about how the changes in the business environment change what the firm needs to do.
Thus, the strategy should clearly be reevaluated any time that a significant change occurs that will affect the market in which your firm operates.