But what decade did it become abundantly clear that the society was indeed high control religion or even labeled a cult?
I know that when I first heard of the JWs in the early 80's, I was told they were a cult by some friends, which seemed a common view.
From the perspective of what I have learned and experienced, your question could be stated differently. Cult is not necessarily a label, but it's an effect, something in which some organizations and groups evolve into. I can't speak for CT Russell, but I have the feeling that from the get go, he loved controlling other people's lives for his own gain. Hence, the cultic effect might have been present since the beginning.
A good indicator of how (not sure about when) it evolved into what it became/has become could be looking at the history (if it was ever documented) of their implementations of rules that started aiming at controlling people's lives and independent thinking. In fact, that very concept of them attacking independent thinking can be a good indicator. If anyone can research and see when that nonsense popped up in their teachings and literature, that might give you an idea.