Does your organization set limits for how long a board member can serve? Most groups do something like, three terms of three years. There is also a case to be made for setting a term for each position. How long can a member be the chairperson? How long should someone be the treasurer? What about chairing some committees? I see a term of about two years for each of these positions.
If you don’t have a term limit it can be difficult to get new people into new roles. Groups benefit from change and you have to plan to make it happen.