This doesn't really sound like a configurable timeout in MWG. Since I am not familiar with WCCP I am not sure... I would recommend to file an SR with support and provide them with a packet capture that shows the problem. That should help us to understand what happens here and find a solution.
This sounds like an issue with your Authentication TTL being very low (3 minutes) and SSL scanner off.
Check out the auth guide regarding hard and soft timeouts: