For all impatient, let me tell you how the update went down on my prime.
First time I pressed "Check updates" it told me there is an update, check the notification area. Then, after a few minutes it said the file download failed, try again later.
Second time I tried, it went the same way, but this time the button remained disabled.
I waited 30 minutes and it was still disabled, but the message about checking the notification area was there.
I turned the device off and rebooted, but could not bring the button back to be enabled. I kept looking in the System update page every 20-30 minutes, but nothing changed, and there was no new notification.
Then, out of the blue, it started downloading the new version and proceeded to install,all went smooth, finished in about 30 minutes, the longest part was when it checked all installed applications.
I think that when the button goes disable it means you are put in a queue, probably ASUS servers are overloaded with updating all of us more or less at the same time. When your number comes up the download will begin automatically.