Session State & Authentication timeouts are in minutes not seconds. So, you should have
<sessionState mode="inProc" timeout="30" ></sessionState>
<authentication mode="Forms">
<forms loginUrl="~/Login" timeout="30">
</forms>
</authentication>
Also, you should be aware that the way this is setup, the authentication will timeout 30 minutes after it was granted while the session will extend the 30 minutes to be from last access. To make these two closer to syncing up, you should add slidingExpiration="True" to the forms element.
If, after these changes, it is still logging you out after a few seconds, take look at:
- Is the cookie getting created in the website? It's name will be .ASPXAUTH and it should be a session cookie.
- Are you closing the browser when the timeout occurs?
- Do you have multiple applications using the same authentication method?