I'll be frank here. I'm a writer, not a graphic designer or web designer. I just sort of hammer words down onto a page, and hope they look pretty when they finally get published. Even so, the other day I had a thought.
See, the Internet is all about making a great first impression - about convincing someone in a manner of seconds that your website is where they want to be. In order to do that, it follows that you website has to look good. Otherwise, how else can it pull people in?
But having the same look all the time, that gets stale, doesn't it? Every now and then, shouldn't you spice things up in order to keep old users engaged and get newer users interested? Every now and then, couldn't every website use a good redesign?
Yes and no.
See, provided your website's appearance and layout appeal to your target audience - provided you're getting a decent conversion rate and your site conveys something about your brand - there's actually very little reason for an update. How your site looks could, in many cases, be an integral part of your branding.
There are exceptions, of course. Restaurants and clothing outlets may want to change their appearance based on the season, retail outlets might occasionally kick off a big sale; you get the idea. In short, there's really no concrete means of determining how often you should change your site's look.
At the end of the day, I feel like it's probably more a matter of 'why' than 'when,' anyway. You need to focus on how users are engaging with your site - metrics like clickthroughs, time on site, bounce rates; you get the idea. If they're particularly low, it could indicate that your site doesn't look good (though it could indicate other things, too).
So...basically, it's entirely up to you whether or not you should redesign your site.
I think Idea Launch's Byron White probably put it better than I can:
"Across industries, I have yet to discover a tried and true formula for how often you should change your website, including its landing page," explains White. "Prior to changing your landing page website's design, it's important that you track its success through your conversion rates. While changing your landing page design can lead to higher conversion ratings, the old adage is still true: if it ain't broke, don't fix it."