I just finished a 5-minute, 2-reboot pre-update update to prepare my Nokia Lumia 1520 for Windows Phone 8.1. It's currently downloading the 8.1 update (which appears to be fairly large, but I'm on my home WiFi, so size doesn't really matter).
I'll keep you posted.
>>> edit <<< my download was at 88% and then a small thunderstorm caused a power blip - lost my DSL momentarily, so I had to start over. I'm back to 66% and counting.