Sorry MBM, thats not correct.
The answer to this question depends on the type of authentication you're performing. If you're using Direct Proxy Auth then the answer is, there is no "session duration" because every single connection must be authenticated.
If you're using Authentication server, you can set the TTL in the "IP Authentication server" settings. The default is 600 seconds.
For more information see the best practice, in there I have a section on increasing the TTL:
I didn't get the question right.
Thanks for advice.