I saw a lot of people wondering when the spin off would take place. You have to look into the roles that are planned. In the future series, we will see Prince Daemon Targaryen who was the grandson of King Jaehaerys I Targaryen. So according to the chronology of Game Of Thrones we will land in the reign just after King Jaehaerys I Targaryen. The reign of Viserys I Targaryen. That's either that or we will land just at the end of Jaehaerys' reign.
And for more information, Viserys I Targaryen is the eldest son of Baelon Targaryen. And as for Baelon Targaryen he was the fourth child and third son of King Jaehaerys I Targaryen.
To conclude, we will certainly land in the reign of Viserys I Targaryen. This means that we will already be at the fifth Targaryen King of the Seven Crowns. So between the years 100 and 129 after The Great Conquest.
