This is a General question asked in all  the  Interviews

You start testing once you are clear with the requirements 
amd the documnets are ready

You stop testing depending on the following factors

I am continuing The answer You stop testing when
stop testing these are the main factors.

1. Reduce in defects.
2. Increase in Testing cost.
3. Delivary dates.

We need to check that

1) Whether all the Bugs which were Assigned to developers 
have been fixed .
2) All the documents pertaining to testing have been 
prepared.
3) all the bugs which were in Previous releases have been 
retested .

when bug rate falls below a certian level
 when deadlines are reached
 when all the testcases are passed with a certian rate of 
passed testcases
 when testing budget is depleted
  and teamleader says to stop testing
   in all these cases we stop testing

we will start testing when we have software requirement 
specification, enough testcases,  and build

we will stop testing 
i)when all major seviority bugs are solved 
ii)when budget is going to complete
iii)when project time s going to complete

why no hv mentioned abt the Test Plan -Exit Criteria.
while Test Plan is done we use to mention the Exit Criteria 
Right,we shd Indentify that Exit Criteria also to Stop 
Testing know?

When to start:

 1)When build is ready
 2)Environment is setup
 3)Test cases are executed
 4)Resource should be ready

when to stop:

1)Based on no. of test cases executed
2)No. of defect-reach certain low level
3)Based on release date

The following are the list of items that are required to be 
Start the QA Phase.
.Complete Unit/ Integration Testing.
.QA Test Environment SetUp Completed.
.QA Test Plan Preparation Completed and approved.
.QA Test Cases Preparation Completed and approved.

The following are the list of items that are required to be 
Stop the QA Phase.
. All Planned System Test cases have been executed.
. No Critical Bugs.
. Defect log should be updated.
. QA Test Summary Report Completed.
. QA Sign off.