As soon as personal testimonies, video and photographic evidence and more started to become available, when it wasn't just rumours and heresay, but actual proof of what was happening, how long was it for the denial movement to start? And what was the nature of the early denial argument?
Arguably, you could say with the Germans themselves towards end the war.
When it came increasingly clear the war was going to be lost, Himmler began the process of clearing evidence. Camp commandants were ordered to destroy paper trails, Gas Chambers, and ovens. Most crucially, the liquidation of the camps and the Death Marches were orchestrated so that no Jew would be left to testify against their crimes.
After the war, there was a level of denial in the German population. As the Allies began their process of 'Denazification', some Germans living close to Concentration camps and transit lines claimed to know nothing, while their neighbours had no doubt as to what their purpose was. This is illustrated very well in 'What we Knew' by Eric.A.Johnson