what is the time limit on the "criminal" prosecution for immigration fraud by the government?

There are several crimes that could be called immigration fraud, including passport fraud and illegal entry into the United States, among others, The statute of limitations is generally five years but if the offense is "illegal entry," the government can charge the alien within five year of his or her entry into this country or within five years of the alient being "found in" the USA -whchever is later. Being found in the United States means being discovered by the authorities here with knowledge of the illegality of the alien's presence. See United States v. Thomas, 492 F. Supp. 2d 405 (SDNY 2007).