how can i set session expire time in asp.net(C#)

Hi all
I have many of sessions, and i want to make the session expire time as long as user use the website, so how i can give a expire time for one of the sessions the below code is showing the session expire time but how i can specified the name of the session 

<configuration> <system.web> <sessionState timeout="20"></sessionState> </system.web> </configuration>