Download
advanced Config. Install it, open it up, click on menu\more settings\connections, and check the box "disconnect after:". Then put the amount of time you want for it to automatically disconnect below.
Then reboot.
After this is enabled, it should be off most of the time that you connect, unless you have programs activating the internet. If you have programs activating the internet, then you will need to take a close look at what you have runnign and other settings. the programs that come to my mind were already mentioned. You can also get a task manager program such as dotfred's task manager.