Installation took around 20 minutes (migrating usually takes the longer part) on my 1520. It was a fresh downgrade so no apps are installed. The GDR1 update was installed faster with Cyan than it did on DP though, only around 10mins.
Guys please mention your internet connection too. It will be a better understanding of the scenario if it could have been estimated how much time it takes based on the speed. I have a very low speed connection (512kbps) and I need to understand the scenario. Please guys.