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 A site that looks outdated won’t inspire confidence in your prospective customers.