Power management settings dialog says to blank screen after 30 minutes when on AC power, so I think that's where the 30 minutes comes from.
I have opened a new bug 591699 with more details and the exact steps to reproduce (this time using i386 instead of amd64, with same results).
The bug I'm observing is definitely connected to X screen idle blanking, and possibly ethernet I/O at same time.
Power management settings dialog says to blank screen after 30 minutes when on AC power, so I think that's where the 30 minutes comes from.
I have opened a new bug 591699 with more details and the exact steps to reproduce (this time using i386 instead of amd64, with same results).
The bug I'm observing is definitely connected to X screen idle blanking, and possibly ethernet I/O at same time.