Probably at the minimum, you should look into a website overhaul every 2 years or so. There’s a number of reasons behind this:

Technology changes fast. You don’t need us to explain it, right?
Device trends change. About seven years ago, it was still rare to see large numbers of visitors using mobile devices. But now the majority of traffic is smartphones, tablets, and pads. You have to be sure those devices can access and easily navigate your site by making your designs responsive.

Security is a constant battle. If your blog software is more than 12 months old, chances are it’s already vulnerable to exploits and attacks.

Styles and trends change fast too. Just look at a typical page even from, say, 2008. It looks ancient now, because the web evolves so fast. A site that looks outdated won’t inspire confidence in your prospective customers.

SEO. In 2 years time, numerous updates to Google and other search engines completely stack the deck against sites that use older methods of optimization.

Finally, don’t forget link rot. Sites die, old pages move, and nothing frustrates users like a page full of dead links.

In addition, there may be reasons why you’d want to at least freshen up your website’s architecture as often as every year. That would be if you’re serving up a lot of multimedia content – video, audio, or Flash. New devices and platforms tend to be optimized and updated for more recent content.

Another reason might be if your site is heavily dependent on outside sources, such as social networks, APIs, or plug-ins such as shopping cart apps and commenting systems. The trouble with this kind of site is whenever anybody in the tool-chain updates, everything else gets broken until it’s patched. Get in touch with slinky web design today