To stop testing in following cases:

Hang up or system crash found

All the testcases were passed

No resources left for testing

All the known defect are fixed

All the unknown defect are minor

Deadline pressure are upon us

Volume and regression testing coducted sucessfully

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 (release deadlines, testing deadlines, etc.)
•    Test cases completed with certain percentage passed
•    Test budget depleted
•    Coverage of code/functionality/requirements reaches a specified point
•    Bug rate falls below a certain level
•    Beta or alpha testing period ends