How do you know it is sufficient testing?

Every company has entry and exit criteria. When we test applications, we refer to exit criteria. When we are about to finish testing, then the QA Team (QA Manager) refers to the exit criteria (exit criteria tells the level of defect that you can be comfortable with before it goes to production. For example, there should be ZERO critical defect, ZERO high level defect, ZERO medium defect, 1 Low level defect, all the test cases must be 100% executed etc). Once the exit criteria meet the requirements, then the software is considered to be sufficiently tested.