No one knows that except Microsoft. You need to wait for it patiently that's all you can do.
I would recommend using the NSR tool to directly install a fresh copy of WP8.1 with Cyan firmware. The process is really easy that any lay person would understand. If you have any important data make sure you take a backup.
Downgrade from dp to get cyan. Use Nokia software updater to wipe your phone and the update to cyan. Make sure you backup all your important stuff, just in case. Cyan was out for 520 before any other model in India.
Downgrade from dp to get cyan, I would have rather wait bcoz from pc to update its dangerous with my little speed of internet512 kb per second... What if my phone stops update in between due to connection... It must of no use.... Waiting.....