Japanese are unlikely to win outright. Their armies had stalled in China, and they were not able to economically benefit from the areas in China they had occupied. From 1940-1944, the Chinese and Japanese had adopted a quid pro quo over large amounts of their areas. Neither pursued the war diligently, because neither could afford to. This does not mean there was inactivity or lack of big battles. There were. However, China did not have the capacity to engage in large scale offensives after 1939/1940, and Japan did not have the will to mobilize their society entirely to knock out the Nationalists because of the cost versus the gains.
None of this changes should the US administration refrain from helping China more, or if no European war breaks out.
A likely scenario is this: British complete Burma-Yunnan railroad in 1942 allowing the Nationalists a secure route to receive supplies. Chiang slowly rebuilds his army. Japan refrains from escalting the Chinese war out of concern that Britain and France will step up any support for the Nationalists. Various battles happen in 1940-1945 that keep the stalemate. By 1945/1946, Chiang has sufficiently built up the Nationalist Army that it can begin winning battles and inflicts a substantial defeat on Japanese forces, and for the first time truly reconquers territory. Japan responds by sending additional reinforcements, instituting a new stalemate. The conflict likely remains frozen for many years in this pattern until China gains enough land to sufficiently build up a modern army (will take many years) or a new government in Tokyo agrees to leave China in a negotiated peace. The resolution may not happen until well into the 1950s.