So first I will link to an earlier answer I wrote here that attempts to address the question. I'll summarize a bit below.

It's important to note that opinion polling was in its early infancy in the 1930s, and even in a country like the US it still made big mistakes. The Literary Digest magazine had been conducting polls on the outcome of presidential elections since 1916, and got them right - until it incorrectly predicted a landslide victory of Alf Langdon over FDR in 1936. This happened because the polling techniques and sampling had serious flaws compared to modern techniques (it basically asked its readers to mail in "ballots", and therefore could best be compared to a modern online poll). The first scientific public opinion poll was conducted in 1935 by Dr. George Gallup (interestingly, he found that a solid majority of Americans had negative attitudes towards the New Deal, but also correctly predicted that FDR would win reelection in 1936 by a solid margin).

And that's all in a country with functioning democratic institutions and a free press, none of which existed in Nazi Germany. The best we can really use during that period and even the Weimar era are approximations, like vote shares and totals for the NSDAP. It's very interesting, for example, that the March 1933 Reichstag election (which was a multiparty election but subject to massive violence and intimidation of voters by Nazi party members) gave the NSDAP its highest vote total and share ever: which was 43.9% of the vote, and 17.3 million out of 39.7 votes from an eligible voter base of 44.7 million.

Another metric might be Nazi party membership, although this has its own pitfalls, as it became increasingly tied to government roles as the party and state became intertwined after 1933, and isn't necessarily a reflective marker of beliefs. Party membership stood at 2 million or so in 1933, rising to about 5.3 million in 1939, and 8 million by the end of the war. So even by this metric, some 3 out of 4 Nazi party members hadn't been party members before the Nazi seizure of power.

To quote myself on the postwar occupation-era surveys:

However, the Office of Military Government, US (OMGUS), the military occupation government for the American zone of Germany in 1945-1949, did attempt to gauge what popular attitudes towards the Nazis were like, and the results are interesting.

Only 12% of Germans said they supported Hitler at the end of the war, with 35% saying they never trusted him and 16% saying they lost trust of him at the outbreak of the war, in an October 1947 survey.

However, in subsequent surveys, the results seem to be at odds with this: a July 1952 survey found that a tenth of respondents considered Hitler the greatest statesman of the century, while another 22% considered him a great statesman who made a few mistakes. A May 1955 survey found that 48% agreed with the statement that, except for the war, Hitler would have been Germany's greatest statesman. In 1953, 14% of respondents said that they would be willing to vote for a leader like Hitler again.

A consistent round of surveys from 1946 to 1949 asked German respondents to rate their opinion of Nazism (a good idea? a good idea executed badly? a bad idea?) and a consistent average of 47% rated Nazism as a good idea executed badly.

So even after the war, and despite the fact that occupying authorities were asking the questions, there did seem to be a durable minority of opinion that supported or was sympathetic to Nazism. Depending on how you asked the question, a majority of Germans answered that they did not support the regime, but a majority would also indicate that they saw positives to Hitler and his regime. This would seem to support an argument put forward by Ian Kershaw especially that Hitler was more popular as a leader among Germans than the Nazi Party was as a whole.

In summary, based on those surveys at least (which again were conducted by Allied occupation forces after Germany lost the war), you could make the case that there was something like a 15-20% of the population that was pretty solid in its support of Hitler (and by extension the NSDAP regime), and maybe another 25-30% of the population that had much softer and contingent support that had evaporated by the end of the war.

As a final note, I think it's important to note that the idea of "full fanatical support" is as much a production of Nazi propaganda itself as based in fact. The NSDAP regime wanted to portray itself as completely supported by the German people - all those adulating crowds saluting Hitler that you see in film reels are as much staged Nazi propaganda as anything else (not that they are completely fabricated events, but they aren't necessarily a reflection of the average German person's feelings or actions). Even for those supporting Hitler or his regime, the support wasn't 100% for the whole package of Nazi ideology (which was more than a bit incoherent anyway): many people liked Hitler as a leader, or approved of some of his ideas (including his antisemitism), while disliking the Nazi Party in general.