The changes in RAM usage are gradual and entirely depend on what's happening in the game. There's not a piece of code which tell the game to suddenly crank up RAM consumption on turn 352 The biggest influencing factors on turn time are the number of planets (early on) and the number of ships (later); and these two variables are determined by a bewildering array of possible outcomes.
Forester's post is useful because he points out that just by creating the map and getting to the start, he's using up 60% of his 16 gig of RAM. He can't really say anything specific beyond that point, though, and the further into the game he gets the more irrelevant his results become. The turn time at turn 352 is automatically anecdotal - in his next game, it might take 400 turns before there's enough stuff going on to reach the same slowdown; in the game after, it may be 200.