Skyrim was officially revealed on December 11, 2010 That gave it roughly an 11 month marketing runway, and it was more than enough to build hype.
RDR2, a game with a comparable amount of anticipation had a 2 year runway
With the long wait for TESVI, a proper trailer showing
The setting
The title
A musical theme
A vague hint at the central conflict or hook
Brief cinematic shots and early gameplay
…would be more than enough to generate sustained hype over an 18 month window, and let the fandom imagination roam wild.