When you are declaring Session time out Container consider your session idea time for time out your session.
For example if you set your Session time out 15 Min. And you session use in your action then it will not time out by container. But it last 15 Min your session not use your application then and then your that session time out.
If you are using Session at your action then this session not time out by container. It considers only Session idea time for time out.
So don't worry about declaring it in to Web.xml.