About 4 1/2 years for me. My nostalgia is kind of tainted by the fact that I habitually wince whenever I remember who I was at the tender young age of 13.
To you guys who feel unrecognized, don't! I dunno about everybody else but this place is small enough that I usually recognize basically everybody who posts.
Honestly, though, I really miss the old-guard power users. The place feels so different these days--everyone seems so glum, nobody goes on the big multi-page tangents, there's a lot more arguing and vitriol than I remember. My entire Steam friends list is composed of people who shot offers my way when I dropped my "StEaM iS eViL" schtick; I have not seen some 60% of my Steam friends list in at least a few months. I used to waste afternoons reading Wikipedia articles in between refreshing GD to check for new posts, but I just can't do that anymore--there's hours between posts most days. On the front page right now, there's a thread nobody's posted in since the 23rd.
Also, what the heck happened to FG&RP? When I joined up it was effectively Play-By-Post & Freeforms, but now it's closer to Arms Race & Suggestion Games. Haven't seen a play-by-post tabletop game run there since I quietly let my last Pathfinder game die. Most of the old FG&RP crowd seems to have disappeared entirely, save the FEF regulars.
E: Also, Mafia is horrendously dead. As far as I can tell, there's only enough people to run one game at a time, and it seems like the same cabal of regulars (along with assorted noobs filtering in from fgrp) forms the backbone of every game.