With a POD around 1920 or so, how long can the Weimar Republic last "as-it-is" without falling into some sort of totalitarian regime?
Not necessarily with a PoD as early as 1920. Have 1923 go differently, or mess with global economic trends - a lot can happen.Probably about as long as it did OTL. The Great Depression hit Germany hard just as the Republic was showing signs of life.
In the Nov. 1932 election (the last free one before the Nazis took over) about half the electorate voted for either the Nazis or the Communists - two groups who were explicitly anti-democratic. People were more than willing to end the "democracy" experiment and try something new and totalitarian in nature. Even if you butterfly away the Nazis someone like them will eventually topple the government by the mid 1930s at the latest.