Just as it is not easy to enact constitutional amendments, it is not easy to repeal them. The Great Depression made repeal much easier because the US government was running a massive deficit, and the idea that taxing a legalized liquor industry would help make up for it (without unpopular tax increases elsewhere or drastic spending cuts) became attractive. (The Depression also helped to discredit the argument--made quite seriously by a great economist, Irving Fisher-- that Prohibition was helping to keep America prosperous by improving worker productivity.)
Another reason repeal was unlikely much earlier than in OTL: Al Smith's defeat in 1928 was seen as a victory for Prohibition. This was probably wrong; prosperity, religion, Hoover's good reputation, and the fact that the Republicans were the majority party were almost certainly more important in Hoover's election than Prohibition. Montana provides an illustration: its voters rejected a state prohibition enforcement law by 54.09%-45.91%.
http://tinyurl.com/o6ystyw On the same day, they also voted for Herbert Hoover over Al Smith 58.4%-40.5%.
http://psephos.adam-carr.net/countries/u/usa/pres/1928.txt Nevertheless, Hoover's election did temporarily strengthen the prohibitionist ofrces in Congress, and only the Democratic gains of 1930 and especially 1932 made repeal plausible.
Making the 1928 election a true referendum on Prohibition without other issues like religion may help, but that would require the Democrats to nominate a strong Protestant anti-Prohibitionist, and I can't think of one who could beat Hoover. (The most prominent anti-Prohibition Protestant Democrat in the 1924 race, Oscar W. Underwood of Alabama, had retired from politics in 1926--he faced a tough re-election fight against the Klan-supported Hugo Black--and would die in 1929.) It's also unlikely that there will be a "dry Democrat vs. wet Republican" race. Both Hoover and his leading potential opponents--Frank Lowden of Illinois and Charles Curtis of Kansas--were "dry." So as usual it's hard to make the presidential race into a referendum on a single issue.
All in all, it is much easier to see Prohibition never enacted in the first place than repealed much earlier than it was.