DBA Oracle site suggests setting it to 30 minutes
You can use profiles to set the connect time and idle time with "alter profile" statements. Note that idle_time is expressed in minutes and you can express idle_time for 30 minutes by setting idle_time to 30:
alter profile senior_claim_analyst limit
connect_time 180000
sessions_per_user 2
idle_time 30;
By setting resource limits like idle_time, you can prevent users from performing operations that will tie up the system, and prevent other users from performing operations. You can use resource limits for security, to ensure that users log off the system, so as not to leave the session connected for long periods of time.
The idle_time parameter limits the maximum time a session can stay connected without doing anything. By reducing the period of time an inactive session stays connected, the probability of that session being a victim of abuse is reduced.