I think it's most likely after the end of the Azkaban arc, but before then I was of the opinion that Voldemort was somewhere else and Quirrell was a red herring. Certainly there have been various foreshadowings, but only the sense of doom was actually good evidence, if Eliezer was willing to foreshadow things just to keep us on our toes. But now that we also have drastic personality changes, I'm willing to accept it as most likely.