Generally, a major redesign is recommended every 2-3 years, but it can depend on your industry and specific business needs. Even if you are happy with the look of your website, both the framework underneath as well as any new web technologies may be best served by a major redesign. Areas such as accessibility, mobile responsiveness, and speed can be hard to improve drastically without a large refresh.
Regular updates to content and minor design tweaks are also advisable to keep your website fresh and relevant. For the best search engine practices and keeping the website fresh for return visitors, we suggest at least once a month with weekly preferable. A large number of small businesses don't need to update their main content that often, but a blog with articles related to your market published regularly can drastically boost search engine ranking.