the window, such as it is, for a successful Japanese invasion of even part of India, without major previous divergence, is 1942. They don't have long either, as the monsoon began on May 10 historically and essentially movement comes to a halt until September. By that time, the British can reinforce significantly at least enough to deal with the few divisions the Japanese can spare.
Much of the Japanese Army is still in China or Manchuria, and the Army just doesn't have the divisions
By 1943, the air picture has completely changed as the RAF and US 10th Air Force are powerful enough to keep control of the air, and sufficient equipment including decent tanks has reached India to make any Japanese advance beyond the jungles of Assam and Burma meat on the table for Allied firepower.
Even in 1942 though, the Japanese lack the troops and it would have required a major British collapse as well as overwhelming problems elsewhere that keep the British from reinforcing for the Japanese to even get as far as Bengal. On the plus side, should the Japanese do so, the British can blame the Bengal famine on the Japanese!