There is no simple answer to this question. It depends on a variety of factors, including the size and scope of the con, the resources and skills of the con artist, and the level of vigilance of the people being targeted. In general, however, it is difficult to pull off a large-scale con without being detected. Sooner or later, someone is likely to become suspicious and raise the alarm, leading to an investigation that can uncover the deception.
Of course, there are always exceptions. There have been cases where massive frauds have been perpetrated for years before they were finally uncovered. But even in these cases, it is typically only a matter of time before the truth comes out. So while it is possible to do the big con without being detected, it is certainly not easy.