We often get asked this question and the answer isn't exactly simple.  Our short answer is one to two years max, since that's what major sites go for, and given how fast the Internet changes, that's plenty of time. Here are some common questions to consider:

How will it make me more money?
A website redesign isn't all about looks. The purpose of for-profit businesses is just that, making money. Having a new design every so often will keep the site looking and feeling like a modern website, something other major websites look and feel like. Also, time changes the way visitors navigate websites, and trying new ways (via A/B testing) to convert your visitors even between the redesign periods will help immensely. New designs can also be A/B tested.  This will maximize the conversions of your new visitors.

My website works and does its job.  Why change it?
This may be true, but referring to my point above, it can be improved to increase conversions, sales, or whatever your goals are. Let's take a retail sales website as an example. Five years ago the way you present your products, your filters and checkout process worked for the time, but now the way people shop may be different. You still may get sales, but you are missing out on more by not changing it. A/B split testing can prove this, by releasing a new design or two, and test it with a percentage of your visitors to see the improvement, if any.

Should I have a mobile-friendly site?
Do you have a mobile-friendly website? Chances are you don't. Mobile web usage has increased significantly in the recent years, 148% in 2009 alone. Because of this, having a mobile-friendly site is critical to stay current and not lose out on any potential sales, especially for service and retail websites.

Why should I pay you to do it?
Simply put, we're not just geeks in a dark room programming websites. We are marketers as well, and our focus on every site we work on is how we can bring our clients more conversions and sales.

The warddrobe metaphor
Here's another way to explain why it's best to redesign or revisit your website every year or two. Do you remember that great outfit you had when you were eight? You loved it so much, it fit well and got many compliments. However, now you're older, we'll say fifteen, and while it doesn't mean it wasn't a great outfit, it was for when you were eight.  Not only does it not fit, it's just not the right outfit for you now. Times change, and in cyberspace, change comes fast. If you don't go with the tide, you'll be left behind.