Other than the requirement for a chairman to be elected at the annual meeting every year, there is nothing in law that prevents someone chairing a council for many years but similarly there is nothing in law to prevent a council amending their standing orders to reflect a limitation if they so wish. It is something the council as a whole needs to consider and agree. Regardless of what the standing orders say, decisions are made by the council as a whole so if the council votes to re-appoint someone, that person is reappointed.
The only legal requirement is that someone is actually appointed chair each year at the Annual Council meeting (or if/when the role becomes vacant if at other times).