Intuitively it always seemed like there was a sweet spot. Too short a period and there's not enough time to build hype or (more importantly) fix the issues revealed by Early Access. Too long a period and people forget about your game, and EA players got everything they wanted out of it before launch, so have little reason to come back and boost visibility w/ player numbers.

It's nice to have some hard data to back this up and put an objective measure on where that sweet spot is.