Testing is endless. We cannot test till all the defects r removed.

common factors in deciding:

1.Deadlines(release test deadlines)

2test budget depleted

3.bug rate falls below a certain level

4.coverage of functionality reaches a certain level

When you write test cases based on the requirements, the Test Team has to test and approve the cases. After that, testing should be stopped.

This can be difficult to determine. So many applications are so complex, so that complete testing can never be done. But some common factors in deciding when to stop: 

• Deadlines (release deadlines, testing deadlines, etc.)
• Test cases completed with positive percentage passed
• specified coverage of code/functionality/requirements to be at certain level
• Bug Count falls below a certain level
• Beta or alpha testing period ends