So finally I made it, I have been crowned legendary ! My actual activity is 868. I am glad I did not had to wait very long to become one !
It spent 29 days, 11 hours and 52 minutes to become one and I made 3095 in the meantime. Those numbers might not be exactly right, as I saw it now, and theorically I am one since Tuesday, but still it gives you and the posterity an idea.
Now theymos really needs to add another rank so that I can reach it !
Congratulations.
People are now hitting 1500 and 2000 activity points regularly, so I do think that the idea of a new rank is good.
However, for the average Joe in the forum he'll probably not be engaged for that long and only have a few activity points, and making a new rank for no good reason isn't going to be economically feasible.