Enough testing has been performed when:
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time runs out.
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no more faults are found.
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the users won’t find any serious faults.
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the required level of confidence has been achieved.
Testing is complete when the required level of confidence is achieved - this is the only valid completion criterion among the options. Time running out is a practical constraint, not a theoretical one. Finding no more faults is impossible to prove. Users not finding serious faults is a desired outcome but unverifiable beforehand. Confidence level is the professional standard for test completion.
Testing is considered 'enough' when the required level of confidence in the software's quality has been achieved, typically defined by risk-based exit criteria (coverage targets, defect density thresholds, business risk tolerance) — exhaustive testing is impossible, so testing always stops at some practical, agreed point rather than a theoretical completeness. 'Time runs out' describes a schedule constraint, not a quality-based stopping criterion. 'No more faults are found' is unreliable — absence of found faults doesn't mean absence of faults. 'Users won't find serious faults' is unverifiable and not an actionable test-exit criterion. Only the confidence-level answer reflects standard test-management practice (e.g., ISTQB's exit criteria concept).