First log into your NetScaler, Navigate to your Session Profile you want to change the default Session Timeout. The below screen shot we are extended the Default 20 minute Timeout to 4 Hours (240 Minutes)
Once we change the value on the NetScaler we need to change the Timeout value on the Web Interface Server(s). Screen shot below shows the configuration Screen.
Also note if you are using a Load Balanced VIP for your Web Interface Servers (Highly Recommended if your not). You will want to change the Persistent cookie timeout value to match.
Thanks for reading hope this helps