There's no universal answer to how often a website needs a redesign, but there are specific, practical signals worth watching for rather than picking an arbitrary number of years.
A clear signal is when your site looks noticeably dated compared to your direct competitors. Design trends do shift, and a site that looked current five years ago can start to look neglected next to newer competitor sites, even if nothing is technically broken.
A more urgent signal is a mismatch between what your business does now and what your website says. If you've added services, changed your target customer, or repositioned your brand and the website still reflects the old version of your business, that's worth fixing regardless of how the design looks.
Technical signals matter too -- if your site is slow, doesn't work well on mobile, or is built on outdated technology that's hard to maintain or secure, that's a functional problem a fresh coat of paint won't fix, and is worth addressing sooner rather than later.
On the other hand, a site that's performing well, converting visitors, and reasonably current in appearance doesn't need a redesign just because it's been a few years -- a full rebuild carries real cost and risk (temporary ranking dips, migration issues) that should be weighed against the actual problem you're trying to solve.
A useful middle path many businesses underuse is incremental refresh -- updating specific pages, imagery, or sections that have become outdated, rather than a full rebuild -- which captures most of the benefit at a fraction of the cost and disruption.
Web Design
How Often Should You Redesign Your Website?