Small Diffs and Fast Turnarounds Make Code Reviews Effective, Engineering Guides Say

Many code reviews are rubber stamps, with large pull requests sitting unread for days and then merging on superficial approvals. Google's engineering documentation and peer-review research indicate that reviews only catch defects when changes are small and feedback arrives quickly. Reviewers reportedly should cover under 400 lines per session and limit sittings to about an hour, while authors should split large changes into smaller, independently testable pieces. Google also suggests reviewing within one business day, approving changes that clearly improve code health even if imperfect, and framing feedback as questions rather than directives. The guidance emphasizes time-to-first-response as the key metric over comment counts.