Final answer:
The guidelines for writing test cases are usually documented in the test case template, which ensures consistency in test case detail and length among team members.
Step-by-step explanation:
The guidelines for how test cases should be written, including the level of detail and length, are typically found in the test case template. The test case template serves as a standardized blueprint for all team members to follow, ensuring consistency across all test cases. While a test plan outlines what will be tested and the test approach explains how testing will be conducted, it's the template that provides the specific details on how to document each case, including sections for preconditions, test steps, expected results, and test data. The project plan may contain higher-level project activities, timelines, and resources, but it is not usually the document that provides guidelines on the specifics of writing test cases.