I don't really know how to call it. If it's "the group" with an evolution of members, I'd call it about 7-8 years IIRC. The group has been through some permutations over this time period, with DMing transferring at times between one of three guys - alsih2o, Beale Knight, and Brutorz Bill - and the campaign and even ruleset & genre changing at times. Right now, there are three in the group who have been constant for many years, and two of us go back to the beginning. And in fact, if any of the "missing" members showed back up, they'd fit back in in no time.
That makes it 7 or 8 years. But... me and the DM, while I didn't game with him from the time I left town after high school to the time the above group got together, our gaming relationship goes back to high school. Call it about 31 years. AND - the above group was part of a constant progression of games from the one I was in with Beale Knight in high school.
So... I don't think it's too far off the mark to call it a 31 year group, even though I was on "sabbatical" for a good chunk of those years. In fact, back around the holidays, Beale caught up with a couple of the original group from high school and invited them over to have a hand in a game for a night, and like I say in my first paragraph, man, they fit right in as if they were never gone.
So, on that basis, I'm going to vote 31.
hth