If you made 500 posts in the past 30 days it will be 100%.
That's as specific as I can get, as it makes no further calculations.
So for example, I have a 9.6% activity... meaning I have made (9.6 / 100) * 500 = 48 posts in the last 30 days.
And again, by posts, I mean when your post count goes up. So posts in the bored board do not count, unless it's a RAF Party.