Do You Have Terms for Your Ministry Coordinators?

I have been a member, and a ministry coordinator, at the church I attend for over a year now. One of my tasks has been to gather mandates and documentation on the ministries that are run in the church, and to update them. 

In talking to those involved in various ministries, I have found that some have been in coordinating positions for 10 to 20 years! There is no written commitment or terms of service in any of the mandates so I have been trying to implement the idea of terms (the council does this, but not ministries and other committees). 

Many of those who have been involved for 10+ years really do love what they do and don't feel that terms are necessary. However, I feel it is important to provide a way for those involved to step down without feeling like they have to stay until someone else steps in. In addition, it also gives others an opportunity to use their gifts without feeling like there is no place for them. 

Just wondering how other churches have their commitment to ministries and committees set up?