The expiry time is the same as configured in the Joomla! global configuration (back end).
However when the user logs in from the same working station (say the same PC), then he/she can always log in for a second time, meaning he/she doesn't need to wait for the first session to expire.
See the information under 'usage notes' in the file DOCUMENTATION

I installed Only once plugin and it is working fine.
I just want to know that if a user forgets to logout from his session then by how much time that users session gets over. So that user can login again in the same site??