Given how desperately corrupt Chiang's regime was, I'd be tempted to say that the latest possible date is when he took over the KMT. (Jiang, GMD, mutatis mutandis)
Chiang himself was personally not corrupt. And the KMT regime tended to provide much better governance in the areas it controlled than the warlords it replaced. Only a few warlords could be said to have governed equally effectively or better than the KMT - Yan Xishan, Li Zongren, and Long Yun.
Corruption was bad compared to Western countries prior to the Sino-Japanese War, but it was getting much better. There was real improvement during the Nanking Decade.
However, the Chinese economy suffered greatly because of the war, and especially after the Burma Road was cut, China was no longer self sufficient. Shortages were high. The KMT government did what lots of governments do during such situations - they turn on the printing press and cause high inflation which just ruins the economy even more. It was only really in 1943 onwards that corruption really killed the KMT. However, I doubt few governments in the exact same situation could have done much better.
Chiang's major problem is that he failed to adequately punish those who were known to be corrupt. He knew how close he had come to defeat during the Central Plains War when he prematurely tried to reform the army. He knew he could only afford reform once he was politically secure (by having the dominant military) and favored incompetents and thieves whom he knew to be loyal to competent, honest men who were not.
Yet once he was in position that he no longer relied on the support of regional powers (when he was alone in Taiwan without any warlords), he quickly cleaned house.
It's important to note the terrible damage that corruption did to the KMT's repuation in the later half of WWII and the Civil War, but also note that the level was tolerable during the Nanking Decade and disappeared completely on Taiwan. It's not a simple story.