I don't know if I'm knowledgeable enough to meet the high standard of this SubReddit -not being sarcastic! It's a little intimidating-, but I can take a stab at it.

Most of what I'll discuss is sourced from Thomas Weber's 'Becoming Hitler' that discusses the very question you are asking. For the record, I didn't read that book, I listened to it. Audible is really a wonderful way to educate yourself while exercising but it's not as convenient as the written word when you want to quickly check a few facts or passage! Should have gotten a .pdf.

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Hitler gave a completely distorted account of his life and sought to portray himself as a man who had always opposed Jews and communists right from the start in Vienna because of his experiences. Of course, in Mein Kampf he portrays this as a heroic struggle while afterward it was interpreted as villainous. But whether you are impressed or appalled by what Hitler makes of his youth, you are being hoodwinked. It's mostly lies designed to position himself for the role of the Furher that he was starting to envision for himself at the time.

According to Weber, Hitler wasn't quite the failure as an artist that many think he was and in fact some of his best patrons were Jews. After a low point that saw him living in a homeless refuge, he started to rustle up just about enough business to eke out a living and when he moved to Munich his situation improved slightly and again some of his most generous patrons were Jews. He was no master but actually managing to live off your art is a long way from being a failure and unless you believe that every artist deserve to be pampered and comfortable, it's hard to argue that the art world was especially harsh to Hitler compared to most other artists of the 1910s era.

Then comes WWI and Hitler volunteered and served in the List Regiment. Note that as an Austrian citizen, he had to seek and obtain permission. He worked as what you could call a runner or administrative aide attached to the HQ of the regiment. He was unpopular with the other enlisted men because he was a bit of a loner and had a relatively cushy job (as cushy as it gets near the front) but he was popular with the officers who found him diligent and dedicated. Amusingly, it seems that reports survived that show that while they liked him as a runner, his superiors saw no leadership potential in Hitler. Eh.

He was so appreciated by the officers that at the end of the war he was recommended for an Iron Cross. Contrary to what propaganda would later claim, this award was not tied to a specific act of bravery (as should be the case). Basically, some officers just wanted to give a bonus of sort to a beloved subordinate. According to Weber, the officer who signed the recommendation was a Jew; Hugo Guttmann. He would run afoul of the Gestapo in 1937 and escaped to the US where he would never brag about being the guy who recommended Hitler for an Iron Cross!

So, what do these select few facts have to do with your question? Well, it seems very doubtful that Hitler was a rabid anti-Semite in his early life as late as 1918. He just couldn't have worked successfully with so many Jews, especially his direct superior officer in the army, if he was seething with violent hatred at them. Whatever made him break bad had to happen later.

Based on Weber's work, here would be a rough outline of what probably happened. First of all, Hitler was a bit of an aimless slacker at the end of the war. Based on his researches, Weber identify only two firm belief of Hitler at that point; he was a pan-german nationalist; meaning that all German cultures (such as the Austrians) should unite in a greater German state. Secondly, he was anti-internationalism. That really is an extension of being nationalist, in many way, but essentially he was opposed to forces such as a communist movement directed from Moscow or international finances. However he was not necessarily opposed to socialism itself and would for a while show sympathies for left-wing groups like the SPD (Social Democratic Party).

After the war, Hitler did not demobilize and stayed in the army. As a result he was still in the army when there was a socialist coup in Munich and the army tacitly accepted the new, short lived, government. Not only did Hitler not leave, he found himself elected to the soviet of the army to represent his regiment! I laughed out loud when I got to that passage in Weber's book. If you stretch the fact a bit, you could argue that Hitler served in the Red Army!

https://en.wikipedia.org/wiki/Bavarian_Soviet_Republic

It's a short lived episode but surprisingly convoluted so I won't get into it, suffice it to say that this was Hitler's first foray into politics.

It's after this episode that, somehow, Hitler becomes progressively a radical. Weber refrain from making sweeping claim about what Hitler thought and when. But after the fall of the republic, several things happen and it's around that time that he would turn into the raving lunatic we all love to hate.

The trigger could have been the Versailles Treaty. Germany surrendered in November 1918 but to the man on the street, they did not really know how badly Germany had lost. Through the spring of 1919, the feeling was that either the war had been a draw or that otherwise the terms would not be too bad. After all, the allies never even set foot on German soil (well, the Russian did but they had surrendered to Germany a year ago) so how bad can it be? Very bad, as it turned out and most would only discover it in June 1919 after the treaty.

That the treaty of Versailles was unduly harsh and made WW2 inevitable is being more and more conclusively shown to be a myth. But, it certainly was felt as a shock and as far too harsh and humiliating by many Germans, Hitler very much included. Weber argue that after Versailles, Hitler would become obsessed by two questions: Why did this happen and what should be done so that it never happen again?

Clearly, at some point in that period Hitler settled on the 'Stabbed in the back' (by jews and leftwing politicians) myth to answer the 'Why'. So a credible answer to 'when did Hitler start hating the Jews so virulently' could be 'When he bought into the 'Stabbed in the back Myth' some time after the versaille treaty' through the month of July 1919. Unfortunately, it's not like Hitler kept a precise and honest diary at the time so we can only guess.

At about the same time he had become a propaganda officer for the army after the soviet republic fell (through some street fighting with the Freikorps if memory serves). Hitler's regiment had stayed pretty much on the sideline and so did not fight against the Freikorps but it did not help them either and now that the government was back in conservative hands, he must have been worried about his standing. So he gladly volunteered in a new program to train propaganda officers who would promote proper (right wing) values to the rest of the army. In that context he would meet many teachers that promoted a not quite coherent set of ideas designed to fight off communist propaganda. He took some of their ideas and rejected others. It's hard pin down the exact role it played in his radicalization but it is concurrent to the process and at the very least it gave him some useful rhetoric tools to express his opinions so it is a very important piece of the puzzle. Some of these teachers would find themselves in the Nazi Party down the line, many would find themselves in Nazi camps as political prisoners.

Around November he would finally meet his destiny; His superior officer sent him to observe and monitor a meeting held by a small political group called 'The German Worker Party' recently founded by one Anton Drexler. That group was virulently nationalist and anti-Semitic. Rather than just observe, Hitler involved himself in a virulent debate with a guest speaker that had triggered his anti-internationalism pet peeve and by so doing he greatly impressed Drexler who gushed praised at him. And as icing on the cake, an older, very respected member of the group called Dietrich Eckart, something of an intellectual and author, took him under his wing and groomed him for leadership. Soon Hitler left the army, joined the Worker's Party that would rename itself the NSDAP (The Nationalsozialistische Deutsche Arbeiterpartei from which we derive the moniker 'Nazi') and he never looked back. The more radical his speeches, the more adoration he received so it feels like a positive (for him, not the world) feedback loop at that point.

Bottom line, as late as early summer 1919 we can demonstrate his actions do not line up with those of a radical anti-semite but by the end of 1919 we have letters from him that proves conclusively that he had become exactly that. The transition seems to have occurred in a few key months between July and November of 1919. He was not yet fully formed in his beliefs, he was not yet a radical anti-communist for example, but that would come soon enough.

To me, all this feels a bit like stories of Muslims youth becoming ISIS fanatics almost overnight. There are probably a lot of parallel to be drawn.

Hopefully that was helpful and reasonably in-depth.