In my web.config I set session timeout as follows...
<sessionState cookieless="false" timeout="99999">
But it seems to timeout after like 10 minutes. No, I don't clear it in code anywhere.
(I'm using my own security and I have session("LoggedIn") as True. If it's False it's kicked out to the Login page. So, after like 10 minutes it kicks the user out to the Login page.)