I think this differs a lot between communities and use-cases. In my experience looking at 50+ communities we support I spot the following patterns:
There are two main reasons to care about the design of your community:
User Experience – making sure members can find their war around and the interface is user friendly.
Marketing - creating a attractive visual representation of your community and it’s activity to recruit new users.
1. User Experience
Updating and improving the design in order to improve the experience is crucial and if you have the resources this should be an ongoing task. Communities are complex and broad enough the focus on a part of the platform every other period (i.e. shift between improving discussions, sign-ups, categories etc) If you do not have the resources or time to do this as an ongoing task make it a priority to find those resources on a regular (every six months) basis.
A good trigger to spend time improving the UX is feedback from your members – if things work they work, but if parts of your community are left unattended you might improve the steps leading toward that section.
Depending on how important the marketing and attractiveness of your community is to your organization the answer can differ. In my experience this is how many times most organizations do a refresh.
Community as an additional platform
For companies where the community fulfills an additional goal and it’s not the main website or platform, you can afford to keep design minimal and functional. Adding a bit of your branding and logo will be sufficient. To recruit new users make use of landing pages on the main website of your organization and collaborate closely with the marketing department to create a solid and timeless message. Re-evaluate the content every 3 months and refresh that design once a year. Link to that page from your community for people seeking more information. Redesigning your platform itself does not have to be a priority and can be done only when it’s triggered by business goals or feedback from users.
Community as a main website
If your community is both an external channel to recruit new members or even customers as well as your main community platform – design is complex and extremely important. It serves two goals at the same time: help your members around and frictionlessly recruit new ones. However, you should be fine doing a full redesign every 1-2 years BUT ONLY WHEn you keep your homepage and landing-pages in check on a very regular basis. Re-evaluate and update the main content and structure on your homepage every 3 months make sure you do a full reshuffle and redesign of the homepage at least every year.
Last but not least I want to emphasize to not stress to much about the design. A community that is engaging, active and relevant will beat any well designed platform. I see many community managers escape there priorities of managing members and content by spending too much time tweaking features and design. It’s most definitely not the most important part of building a great platform.