Policies can be retired and easily amended so incorporating term limits into the by-laws of your society is recommended to avoid directors approaching term limits easily removing the obstacle to re-election/appointment.   Term limits will ensure that you have continuity of knowledge and expertise while providing an avenue for renewal.  Consider how easy it is to find directors for your organization when looking at the the length of any given term and the maximum number of terms for a director.  You will also need to look at how many directors you want turning over in a given year.   Term limits for the position of Board Chair might be incorporated into your governing terms of reference for the position or philosophy documentation.