Apparently you want to get approved by Apple, so I assume it's not for jailbroken devices.
Short answer, you can't. See this question for explanations : Correct way to poll server in background
Also it seems there has been some "hacks" to try to have a background app running forever according to this question : iPhone - Backgrounding to poll for events
But anyway, you have quite a big chance to have your App rejected if you try it. If the values you need to check is on a server, you should consider to use push notifications.