When to Stop Testing and Testing Completion Criteria

Testing should be stopped when it meets the completion criteria. Now how to find the completion criteria? Completion criteria can be derived from test plan and test strategy document. Also, re-check your test coverage.
Completion criteria should be based on Risks. Testing should be stopped when - 

• Test cases completed with certain percentage passed and test coverage is achieved.
• There are no known critical bugs
• Coverage of code, functionality, or requirements reaches a specified point;
• Bug rate falls below a certain level, now testers are not getting any priority 1, 2, or 3 bugs.

As testing is a never ending process we can never assume that 100 % testing has been done, we can only minimize the risk of shipping the product to client with X testing done. The risk can be measured by Risk analysis but for small duration / low budget / low resources project, risk can be deduced by simply: - 

• Measuring Test Coverage.
• Number of test cycles.
• Number of high priority bugs.