I would say that, 2015 Herd announced serious cut back and a halt to most construction projects, was in reaction to a plummet in contributions they may have picked up momentum a few years before, but because the Governing Body is really stuck in their delusions about the Organization being the earthly branch of God's Universal organization they responded way too late because they were hoping for a miraculous bailout from Jehober, and because their denial is so great negative facts take a very long time to sink in, which is leading them in a relentless march toward a nightmare stage of wishful thinking.
So I would guess somewhere around 2001 with Dateline, Panorama, and other publicized child molestation problems inside the Watchtower that many started to wake up and around 2010 the cash flow in contribution started to nose dive, because many members were waking up and stop contributing anything.
In my case for years before I finally learned TTATT I stopped contributing. So I think because they DF people stop believing,, that many who are attending meetings are no longer believers and do not give any contributions, and account servants who are awake are skimming some off the top.