What are you trying to accomplish?
Are you trying to change the timeout period? According to the documentation that property doesn't change that.
I think you have to set the timeout in the web config:
<sessionState mode="InProc" timeout="5" />
Or you can do it in C#:
Session.Timeout = 5; //Minutes
If you want to do the current session you should be able to do something like this:
HttpContext.Current.Session.Timeout = 5;