Apparently, Enlightenment uplifts them to the tech level you were at when you last selected the option in the Observation Post. But if you switch off of Enlightenment and then back on it retains your progress.
So if you switch to "Passive" when there's a month left on the Enlightenment meter, and then switch back, you'll have a vassal in a month.
Not sure if that still works though, they may have changed it.
I doubt that it has worked like this for several updates because Enlightening a species at Atomic Age or Early Space Age take almost no time at all and if they started at YOUR tech level, even if it was the one where you pressed the button, your empire has no time to advance nearly enough to create a meaningful difference in that small amount of time.