Final answer:
IDS systems should be tested at least monthly or as per the organization's security policy to ensure continuous effectiveness and adaptation to new threats.
Step-by-step explanation:
The question pertains to the frequency of testing Intrusion Detection Systems (IDS) after they have been installed. Typically, after the installation of an IDS, it is crucial to test the system to ensure it is properly detecting unauthorized access or attacks. While there is no one-size-fits-all answer, as the frequency of tests can vary based on the organization's policy and the environment in which the IDS operates, best practices suggest that IDS systems should be tested at a minimum whenever there are significant changes to the network infrastructure, when new threat patterns emerge, or as dictated by the organization's security policy. However, it is generally recommended that at least monthly tests be conducted to ensure the system's effectiveness over time.