It is curious to know how much merit PawGo will have when he gains enough activity to reach Legendary rank.

If I needed the required amount of activity, then I would prefer not to know this figure. Let it be better a random value that introduces an element of surprise. Anyway, a new rank is inevitable for you, PawGo. Sooner or later it will happen. About 3 months later (if I calculated correctly) start checking your rank more closely after updating your 2 week activity cycle.