That is if he gets the required number of merits for the new rank, his activity might be due for the new rank in next 4 weeks or less just like you said but he also needs the merit to move on to the legendary rank.
When people tell you,"
Post in meta dude.. You'll get merits easily..." And then you come here saying crap without knowing anything. Nice.
You could never actually estimate at what level of activity you'd turn into legendary, even before the merit system. People could see the potential activity(the real activity level if an user had posted 14 posts every 2 weeks to estimate) from achow101's
website(which is down indefinitely due to clouldfare) but they could never see when a person would turn into legendary. I got legendary rank at around 1000 activity. It differs from user to user, don't know why only legendary accounts have that floating activity rate.