Sessions are set automatically by the server on page load, and the default life of a session is 20 minutes. If you are overriding this setting (which is acceptable) and you are setting it to ndays (which is highly unnacteptable) then you are putting an incredibly heavy load on your server memory and will cause a whole load of pain for yourself.
It may help our understanding of your situation if you detail exactly what sessions you are refering to and how they are being assigned.
Still have no idea what sessions you are refering to. Can you post a screenshot of the sessions you are talking about? Where are the 'sessions' being stored? If you are physically setting a session variable or persistent cookie then you can delete/expire/unset that session variable based on a time calculation. If the session variable is a server generated session variable then it will expire automatically based on the lifetime setting in the server.