Session is always a most discussed topic in ASP.Net. Either you are beginner, intermediate or expert, you always play with them. If you don't know about the basic of session variable, please read my blog entry where I had wrote a very good quick and short tutorial on session variable (value assignment and retrieval).
Session Timeout is one of the most critical setting. It depends on the application requirement. You could set this property in Global.asax/Web.config. You can set session timeout in any one of them.
Web.config will use following setting. Look at the timeout attribute of sessionstate. You can set number of minutes. The default value is 20 minutes. You can set up to 1 year (525,600 minutes).
<sessionState cookieless="true" mode="InProc" timeout="1">
Let me know, if blog entry about Session Timeout help you out to understand session concept. Your positive comments and questions are most welcome.