How many decades? No more than two. Some researchers recognized early on that Piltdown didn't fit. Friedrichs and Weidenreich had both, by about 1932, published their research suggesting the lower jaws and molars were that of an orang (E.A. Hooton, Up from the Ape, revised edition; The MacMillan Co., 1946). This is what a 1946 textbook shows, several years before the claims for Piltdown were completely falsified.
Yeah, but the point is...it was 20 years--at least 20 years of touting it as factual. There was no caution, no "let's test this out" before we claim it's real. It was "let me jump on the bandwagon so my peers will know I'm in agreement with them and thus obtain social and political capital". It was all an ego race. Nothing ever changes, btw.