Afinal ir plan should be tested at least by performing at least a structured walk-through test and a more realistic type of test, when possible.

The answer is ...should be tested at least semiannually
The testing process does not stop once the final IR plan is created. It should be tested semiannually by performing at least a more realistic type of test and a structured walk-through test. 

The IR should be tested at-least Semiannually
Incident Response (IR) plans are designed to test company's ability to respond to a security incident.The ultimate goal is to handle the situation so that it limits the damage to the business while reducing recovery time and costs.
Running such tests keeps the IR updated and fit for purpose in the modern age, while also critically helping to identify and fix weak points.

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