This can be difficult to determine. Many modern software applications are so complex,and run in such an interdependent environment, that complete testing can never be done. Common factors in deciding when to stop are:
What is verification?
Testing does not offer any opportunities for career growth.
Testers do not perform well in development projects (or Testers are poor in coding)
No formal training is needed to work in a Testing project... Anyone can be a Tester!
What is Agile Testing?
What is Ad Hoc Testing?
What is 'Software Testing'?
How can World Wide Web sites be tested?
What's a 'test plan'?
What's a 'test case'?