Adjusting Session Times

We just got a client whose requirement is that user sessions expire after 30 minutes of inactivity. Our other clients are happy with not having their sessions expire during the work day (i.e. life is 8 hours). I am using a MySQL database to store the session data. My thought is to adjust the session expiration in the table based on the client currently logged in. Is this a good approach or would there be better ways to do it? And just to clarify: all clients use the same Web site. Thanks! Floyd