The defualt TimeOut value for Form Authentication is 30

There are two type of Expiration of Authentication which is defined using slidingExpiration attribute

1. Sliding
2. Relative / Not Sliding

 in SlidingExpiration the user's authentication expiration time will reset eveytime he hits the server

when the SlidingExpiration is set to false the Authentication will expires when user reaches the timeout limit...

If you want to change the timeout value to be longer  in your local web.config file which is in minutes...

 

<configuration>

<system.web>

<authentication mode="Forms">

                  <forms loginUrl="~/Login.aspx" name="_LOGON" timeout="500000" path="/" slidingExpiration="false"></forms>

            </authentication>

</system.web>

</configuration>