Use the combination of STATUS and LAST_CALL_ET in V$SESSION. If the status is INACTIVE, then LAST_CALL_ET is the number of seconds since the session was last active.

Please tell me where I can find info regarding idle session (ORACLE V7.3.4). We try to control idle time by the PROFILE, but the application (in-house) will hang if not connected to oracle, and we need to kill the user-id in the O/S. Any suggestion?