*after executing all test cases
*after defliting the budget
*at the end of release date or testing dead line date.

As a Tester:
When Bug rate is 0.
or when the software has too many bugs and has severety  
show stopper,testing has to be stopped and it has to be 
fixed.

Testing never ends,
Why because for every change of test environment,
bugs will come 

The following are few of the common Test Stop criteria: 

- All the high priority bugs are fixed.
- The rate at which bugs are found is too small.
- The testing budget is exhausted.
- The project duration is completed.
- The risk in the project is under acceptable limit.

Practically, the decision of stopping testing is based on 
the level of the risk acceptable to the management. 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 testing done.

we can stop the testing by using the following factors:
1.If release deadlines or testing deadlines are reached.
2.Test cases are executed with certain percentage passed
3.Test budget depleted
4.Coverage of functionality reaches a specified point.
5.Bug rate falls below a certain level
6.Beta or alpha testing period ends

1.when nearing to deadline
2.when all the major functionality covered

Once Exit criteria met, we will verify there isno severity1 
& 2 issues, may be we may have severity 3 & 4 defects 
these things we will menction in clousure reports 
before moving to next level of testing development have to 
give the fixes for the respective defects

When almost all the test cases are executed and there are 
no much big defects

Really, testing stops when the Exit criteria (which is 
decided during test planning) is met.

There are many facts involved for Stop testing. 

It is decided based on the project risk. 

the basic essence is "WE CAN STOP TESTING ONCE THE SYSTEM
MEETS IT'S REQUIREMENT"