If you simply can't wait, you can still get the Windows Phone 8.1/Cyan updates immediately.
Well, semi-immediately, because you'll need to revert to 8.0 first, restore from your backup, and then upgrade to 8.1 through AT&T.
It happens multiple times during the day while there using the computer (it’s not like the monitors are going to sleep from inactivity).
I have updated the firmware on the dock 2.28 Rev A I have updated the drivers for the monitors and even replaced the monitors I have updated the Intel video driver to .4377 Rev.
PPSU_1.03.13 for Windows 7 32bit & 64bit / 8.1 64bit / 10 64bit.
ASUS EZ Update V2.01.32 for Windows 7 32bit & 64bit / 8.1 64bit / 10 64bit.
Not familiar with restoring your phone to the original OS version?
I'm writing this up for and in fact just this past weekend rolled back two phones in order to document the process.
The Dell Venue 8 7840 looks great, but the narrow display bezel come at the price of making the tablet quite awkward to hold.
ASUS Mobo_Connect V1.00.27 for Windows 7 32bit & 64bit / 8.1 64bit / 10 64bit.
ASUS Push Notice V1.01.06 for Windows 7 32bit & 64bit / 8.1 64bit / 10 64bit.
If, however, you previously upgraded with the Developer Preview, you will not (today) see any updates available on your 1520.
Microsoft will at some point patch the Developer Preview so you can upgrade (and I suspect this will be an issue with others phones and is not 1520-specific).