Hi @movovijay05,
You can get the list of all active tokens (logged in users) and expired tokens from [0]. This will list out the time when the token would be expired for a particular user.
[0]: http://<host:port>/system/console/tokenmgr
Sling creates a new session for every request, and closes it when the request is done. There is no jcr Session associated with a user session as such.
When user request the site, sling back-end will instantiate a new session for it or using previously opened (Of cause this is a common case for application server). But a lot of opened sessions can impact performance therefore not active sessions must be closed. For close user session after inactivity we should:
1. Open configuration path:
/system/console/configMgr/org.apache.jackrabbit.oak.security.authentication.token.TokenConfigurationImpl
2. Set appropriate value for Token Expiration property (Expiration time of login tokens in ms.):

And to send reminders prior expiring the user, you can leverage Day CQ Mail Service for that. See https://helpx.adobe.com/in/experience-manager/6-3/sites/administering/using/notification.html.
Thanks!!