There is no fixed number of years after which a website needs replacing. More useful signals include a rising bounce rate, difficulty updating content, poor mobile performance, or a visual identity that no longer matches how the business presents itself elsewhere.
A partial redesign — refreshing specific pages or components — is often enough. A full rebuild tends to make sense when the underlying platform itself is limiting what the business can do, such as struggling to add new features or integrations.
Before committing to either option, it is worth auditing current site performance, user feedback and technical limitations so the scope of the redesign matches the actual problem.