When should you stop testing?
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When the allocated time has run out
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When all planned tests have been executed
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It is dependant on the risks for the item under test
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When there are no more faults
Testing stops based on risk assessment and business decisions, not arbitrary thresholds like time or test completion. Factors include remaining risk, cost-benefit analysis, deadlines, and acceptable quality level. It's impossible to test everything or find all faults.
Testing is inherently open-ended, since exhaustive testing is impossible in practice, so the decision to stop is a risk-based judgment call weighing remaining risk against cost and time. Simply running out of time or completing a planned set of tests doesn't guarantee the risk has been adequately covered, which is why those aren't the right stopping criteria.