In Edwards v. Arizona (1981), 451 U.S. 477, at 484-485, the Supreme Court of the United States said that when a suspect properly asserts his right to counsel before or during custodial interrogation pursuant to Miranda v. Arizona (1966), 384 U.S. 436, and the Fifth Amendment to the Constitution of the United States, "a valid waiver of [his right to have counsel present during custodial interrogation] cannot be established by showing only that he responded to further police-initiated custodial interrogation even if he has been advised of his [Miranda] rights." The cops' only hope would be that the defendant changes his mind and initiates further communication with them. Id. at 484-485.
But today the Supreme Court of the United States put a cap on that protection of the defendant because acquiring "voluntary confessions" is an "unmitigated good" that is "essential to society's compelling interest in finding, convicting, and punishing those who violate the law." In Maryland v. Shatzer (2010), 559 U.S. ___, the SCOTUS said the prohibition on further police attempts to interrogate a suspect like Edwards lasts for just fourteen days. The Justices figure that once the suspect is released from the "Miranda custody" and returns to his "normal life" for fourteen days, the "protective umbrella" of Edwards has served its purpose and the suspect is again fair game.
This is true even for those who are imprisoned based on a previous conviction. The SCOTUS reasoned that a prisoner is not constantly subject to Miranda-type custody when they are living their "normal life" in the general prison population. An inmate is only subject to "Miranda custody" or "interrogative custody" when they are "removed from the general prison population and taken to a separate location for questioning." Once they are released into the general population again, they are living their "normal lives" and the State can try to interrogate the "prisoner suspect" every fourteen days until the prisoner quits asserting his right to have counsel present during custodial interrogation.