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:
Why are there so many software bugs?
Give me five solutions to problems that occur during software development
What makes a good test engineer?
What is Software requirements?
What is a test plan?
What is a test case?
What should be done after a bug is found?
What is configuration management?
What if the software is so buggy it can�t be tested at all?
What if the project isn�t big enough to justify extensive testing?