The forms authentication does not handle session timeout. You want to adjust the Session State timeout instead. MSDN has an article on the sessionState element of the web.config for more details.
If you are going to be using the Session State, I would read up on the different providers. If you ever want to scale your application beyond a single web server, you will want to use a different session provider than the InProc one. Using an out of process session provider requires objects be serializable which will likely be a breaking change later. Make the change now to save yourself headaches later.