It's important to constantly think about redesigning your site. However, lots of people are stuck trying to decide how often they should redesign their site. If you're in this group, here are some ways to figure out what's right for you. Even if you're not doing a complete re-design, optimizing your site for conversions and a better user interface (UI) or user experience (UX) is always a good idea.
First, you should consider what exactly counts as a redesign? Moving a widget from the upper portion of your sidebar to the lower part is probably not a redesign, but it may improve the user experience. At this level of optimization, your website should be considered a continual work-in-progress. There is always something you can do to improve your site and some new variable you could measure. Every minuscule detail about your site can be measured, tested, and optimized. Just take a look at all the studies people have done to find the perfect color for conversions. FYI, there's no one color that performs best in all situations – rather, you should experiment on your own site to see what works for your audience and design.
Every element of your user interface can be tested. The color, shape, and size of various elements on your site can be coded in CSS and tested through simple A/B testing. This will help you identify which UI works best for your audience and which calls to action convert well. As you experiment with UI options, one of your goals needs to be improving the user experience on your site.
When you find that people are leaving your site quickly or not returning to your site, you may have a UX problem. If people can't find what they're looking for they will leave your site and look for another. Think about the last time you landed on a site and couldn't find the information you were looking for. It was frustrating and you had little patience for it.
Also, if it's too much hassle to use your site then people with get what they came for and never return. A great example of this is anyone that ever used a fake email address – like firstname.lastname@example.org. These people want the free report or other offer you have, but they don't trust you enough to share their real email address. Not all the time, but often enough, these visitors think your site looks a little spammy. Some redesign work could help change this image and increase your conversion rate with real email addresses.
As you tweak your site for increasingly better conversions, you'll eventually run into a common situation. You will know so much about the best UI and UX design for your site that you realize the current site doesn't have the features you need. At this point, you need to redesign your whole site from the ground up. It's a big task, but it's the only way to continue improving the site and increasing conversions. As you approach a major redesign like this, think about all the new optimizations you would like to test and build those into your new design. Then the cycle of measuring, testing, and tweaking can begin again.