A month from now, or so.

Also, developers are told that they should not install iOS 7 on a mission critical device, and should only use it on a development device.

But you can downgrade an iOS 7 device to iOS 6 using iTunes, so I even if iOS 7 did expire, it's not the end of the world.

The OTA update to beta 3 from previous betas still works, so pushing beta 4 via that probably wouldn't be an issue.