I don't know if this is even possible; but, if there is no activity when logged into the front end of the website anyway to log someone out like in the backend? Thinking about using your module on a portal website I just got through developing.
Client just emailed and asked if anyway to do that.
Yup. That's a default feature of Joomla. In the "Global Configuration" area of Joomla, find the "Session Timeout" parameter. Set that, in minutes, to the amount of time you want it to take to log the user out if they are inactive. The default is 15 (minutes) I believe.
That setting works for the front and back-end to determine when the user will be automatically logged out.
Hope that helps,