in web.config file find <sessionState> node:
<sessionState mode="InProc" stateConnectionString="tcpip=127.0.0.1:42424" sqlConnectionString="data source=127.0.0.1;
user id=sa;password=" cookieless="false" timeout="20" />
default is 20 - change it and that's it
And also can try the followings:
// Create a new ticket used for authentication
FormsAuthenticationTicket ticket = new FormsAuthenticationTicket( 1, // Ticket version .
DateTime.Now.AddMinutes(60), // Date-time to expire