The session will expire after a preset time - if you are storing login information in the session, then at that point the user will be logged out (this time is reset each time the user posts back to the site)
You can change this timeout timer,
but you should not extend it from it's default of twenty minutes, as it uses memory for each session:
http://msdn.microsoft.com/en-us/library/ms525473(v=vs.90).aspx[
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Instead, have you considered storing the info in a Cookie - the cookie set to expire when the browser instance is closed?
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