How often should changes in leadership occur?
Aside the exception of a challenge to the present leadership, in a vote of no confidence, in the current people in charge. A regular cycle of 4 or 5 years seems a reasonable amount of time. However, one must take care to pay attention to long term views and not just transitional current periods, each leadership team must focus on the future generations of the community and the world as a whole. Not on what might be temporary discomfort.
Leadership, in this sense mean elected officials, or people on committees. there are expert roles that could be exceptionally hard or impractical to replace regularly such as head chef, doctors, accountants, solicitors etc. etc. These unique positions need roper clarification as to what positions require “expertise” i.e. a Chartered Accountant and/or just experience i.e. someone balancing the books using Sage.
Other possibilities include a pool of the most experienced practitioners in a domain (assuming such a group exist) are chosen either at random or each has a cycle of holding the relevant position before ceding the position to another.