No more than a year I'd say.
Anything more than that, and the hype becomes dormant until launch is super close, and speculation turns into degenerative rumors that 80% of the time lead to nowhere and become completely irrelevant by the time the game comes out.
"Just for comparison Uncharted 4 was 20x bigger than Splatoon 2. This shows the huge difference between Sony's first-party games and Nintendo's first-party games."