Recently, concept art for Game of Thrones Season 7 revealed that the final scene will involve a White Walker and a cast of characters including Jon Snow, Cersei, and Tyrion. Although this presents all manner of open-ended possibilities for how it might play out onscreen, it clarifies one major aspect of the story: exactly when the White Walkers will come.
The scene will take place in the King’s Landing location of the Dragonpit, a ruined facility that used to house Targaryen dragons. It’s long been a part of the world, but it has yet to appear onscreen. Allegedly, these disparate characters will gather there because someone — presumably Jon Snow — will bring in a Wight in order to definitively prove that White Walkers are not just a legend.
If people like Daenerys and Cersei still need proof of their existence by the end of Season 7, it stands to reason that they will not fully invade Westeros until the eighth and final season. While this makes sense on a storytelling level, there was always the possibility that the invasion would happen sooner. After all, Bran Stark’s scenes in Season 6 show them rapidly encroaching. But much like Winter, it looks like the White Walkers will still take many episodes to come.