I assume you have two different types of authentication setup: direct proxy authentication -- for proxy users, then authentication server -- for WCCP users.
You would need to adjust the Authentication server ruleset. This is done under Authentication Server (Time/IP Based Session) > Check for Valid Session > You can adjust the value under "IP Authentication Server". It's called the Session TTL.
Pretty ironic, I was just reading the apple forums 5 minutes before seeing post:
They were complaining about similar thing!
I was half tempted to start poking around in those settings (or similar) to see if I could obtain the desired results. I knew that it was one of these rulesets - thank you for setting me straight
Hmmmm, interestingly enough it appears that the issue I was initially speaking of with iOS6 is in fact different - While the timeout issue with WCCP has been resolved (thank you Jon) this new iOS decides to lose its current SSID that its attached to... (Maybe that link you posted warrants some further investigation...)
Thanks again though