It's a gradual roll-out. They do something like 1-2% of devices in the first 24 hours, check for error reports, push out another 25% of remaining devices, repeat, and the number increases until they've covered everybody. It might take a week or two. That's how they prevent bricking everybody's device at once(iOS 8.0.1, I'm looking at you). I think the check for updates button only works once a day, per the same Google engineer that shared the rollout process.
Is your Nexus 7 a Wi-Fi model or LTE model? They're only updating the WiFi tablets at this time. The 3G/LTE models will come later, probably at the same time as the 4.