When you purchase a new copy/or a used one that has all the inserts intact you get the voucher. The voucher is good to get you access to the demo which starts 7/3/12 for Xbox. You can purchase the game anytime and you should have access to the demo. I'm not sure if the demo itself is a limited window offer as there were no mentions of it being a limited time demo.
I doubt the demo will be limited since it's planned to be available to everyone else a month later. Capcom will want to keep as much hype going as possible until release and taking down the demo before then would only hurt sales.