Well, it's kinda impossible to know for sure, as anyone not targeted by the Hitler regime as enemies was still discouraged from publicly, or even privately, disagreeing with the party.
But there was likely some poll or support data available to scholars at different points that would give an idea of actual support at those points.
I don't have those details, and am not who you were asking, but I did hear support dwindled as the US entered the war. And, logically, it was probably higher when he was elected then when the war started getting real nasty.
Most people start not liking their leadership when their leaders actions cause them personal distress.
No, I assure you, I am a loser... But I never supported Trump in anything, and certainly not as a politician.
I would support however his effort to furtilize a random field in Montana with his corpse. But nothing else really.