Hi ,
You can do this as below , What is your OS version ??
what is your mode of authentication ?? Global /transparent
http://www.cisco.com/c/dam/en/us/td/docs/security/wsa/wsa7-7-5/user_guide/WSA_7-7-5_UserGuide-book.pdf
Configuring Global Authentication Settings
Step 1 On the Network > Authentication page, click Edit Global Settings.
Table 20-10 Explicit Forward Proxy Mode Authentication Settings
Credential Cache
Options:
Surrogate Timeout
This setting specifies how long the Web Proxy waits before asking the client
for authentication credentials again. Until the Web Proxy asks for
credentials again, it uses the value stored in the surrogate (IP address or
cookie).
Note that it is common for user agents, such as browsers, to cache the
authentication credentials so the user will not be prompted to enter
credentials each time.
HTH
Sandy
