We've been shown two visions of the throne room and one of King's Landing.
Bran had a vision where he saw a dragon's shadow flying over King's Landing.
https://youtu.be/8gh0-4hOzyw?t=1m20s
In the same vision he also saw the Red Keep destroyed with snow falling on the Throne.
https://youtu.be/8gh0-4hOzyw?t=1m14s
Dany also had the same vision of the Throne.
https://youtu.be/l7VuGknEfQY?t=53s
So what I believe this means is that when Dany lands and attacks King's Landing, it will mostly be intact, aside from what Cersei did but her attack could possibly destroy the Red Keep. At some point after this, The White Walkers will come and with them they will bring the Long Night, completely enveloping King's Landing in darkness and snow.