see attached for how to set up enhanced auditting. Switch it for logins and then you should have an entry each time someone logins to SD and then you use a filter on the audit log to get the information you want.
If you just want to know which account is not being used for a specific period of time, you may try this workaround (but i am sure that its a tidious and no one would like to do this also not sure whether this solves your issue).
Go to Service Today, create a customized view where the filter would be Not Registration Modified by and registration Modifed (provide the period that you want to check)
this might give you some idea on the usage provide that person modified any record in ovsd.