While there's no exact formula for determining how often a website needs a redesign, it's still helpful to note the timeline. If you built your site more than three years ago and it's never been redesigned, there's a good chance that you could benefit with some upgrades and changes.
New isn't always better, of course. However, it's undeniable that our whole society and the tech sector, in particular, tends to favor the new and improved. Just as clothing fashions change annually, new design trends, fonts, features, and themes are always changing the world of web design. So if you've had the same look for a while it might be time to at least consider some upgrades.
If you lag too far behind contemporary website design practices, your visitors will notice, even if they aren't conscious of it. People form quick, often unconscious impressions of a website that help to determine how much time they spend there, what actions they'll take, and whether or not they'll return. Having an attractive, user-friendly, and modern-looking website helps to stack the odds in your favor.