How do you change session timeout in IIS 8.5?
In IIS, open a list of your sites and select one. Then select "ASP" in the site properties under the "IIS" entry
Now just change the voice about "TimeOut", the time you prefer and you're done!
Finally check your Application Pool's idle timeout that application pool idle timeout too matches your session timeout.
Yes, that is the session timeout setting in the "Session State" section.
You can also control it programatically in the "web.config" file. Please see How to set session timeout in web.config. IIS Manager will change that for you with your current method.