A rather fundamental question, I suppose:
At what point did the German military leadership begin to think that they had lost the War?
And what plans had they in place to do anything about it?
That was the question. The answer is, Hitler never had the unquestioning support of everybody, especially the career soldiers that made up the high command. But, he was not an easy guy to reason with. At some point, certain elements decided he had to go, and if he wouldn't step down gracefully he needed a push.
When exactly this took place is hard to say but there were 35 KNOWN assassination plots, many of them by highly placed officials with everything to lose.
So, the answer seems to be that a lot of people saw his policies from the first, as a disaster waiting to happen. But, Stalingrad is usually taken as the turning point. Whether they recongnized it at the time or not.
The majority gladly fallowed him! Admiral Doenitz was one such follower.