I do agree with you “Reviews can be formal or informal, require a well-prepared agenda, and can last hours, days, or even a few weeks.” However, this is not always the case. At the company I am currently at we have something called out in our design review SOP called an abbreviated design review. This type of review is usually only one or two meetings at most. They are generally used for changes that provide enough justification to not require V and V protocols. Now if something is discovered to not necessarily meet justification in the meeting and you discover a V&V is required. A full Design review must be held.