How do you know when to stop testing?

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…
Deadlines, e.g. release deadlines, testing deadlines;
Test cases completed with certain percentage passed;
Test budget has been depleted;
Coverage of code, functionality, or requirements reaches a specified point;
Bug rate falls below a certain level; or
Beta or alpha testing period ends.