Any period of isolation longer than 15 days should be considered abusive, according to Juan Méndez, U.N. Special Rapporteur on Torture. For juveniles and people with mental illnesses, solitary confinement "always constitutes cruel, inhuman, or degrading treatment and even torture when applied to these categories." Méndez has called for a ban on prolonged or indefinite confinement.

At least 46 states use some form of administrative segregation, which is confinement that typically lasts at least 30 days, but can last indefinitely.

In New York state, some inmates spent an average of five months in "extreme isolation cells" during a three-year period from 2007 to 2011, according to a report by the New York Civil Liberties Union. Officials issued more than 68,100 sentences for violating prison rules during that time. In February 2014, New York agreed to broad reforms to scale back its use of solitary, including banning its use for juveniles and pregnant women. Under the deal, the state will also impose sentencing guidelines for all prisoners, for the first time with a maximum time limit.

Inmates sentenced to solitary in Colorado spend an average of 23 months there, and are often released directly from "ad seg" to the streets. In 2013, Tom Clements, the head of the corrections department who had worked to reform the practice, was murdered at his home by a former inmate who’d recently been released from solitary. In February, Clements’ successor Rick Raemisch spent a night in solitary, and said in a recent New York Times op-ed that he was committed to continuing those reforms. "...[D]oing anything less would be both counterproductive and inhumane," he wrote.

Bobby Dellelo was sentenced to life in prison for his involvement in the shooting death of Boston detective George Holmes in 1963. He escaped from prison and was recaptured and sentenced to time in solitary, where he spent more than five concurrent years in a cell he would later describe as "12 feet long, 8 feet wide, and 10 feet high." He was confined alone in his cell for a minimum of 23 hours a day but was allowed a radio and television, which he claimed would talk to him. "Static wasn’t just static," he told the Boston Globe. "It was speaking a language. It was speaking to me."

An estimated 25 percent of the population at Tamms Correctional Center in Illinois -- a supermax prison built to hold the "worst of the worst" -- spent 22 to 24 hours a day in isolation for more than 10 years, before the prison closed in 2013 due to budget cuts.

Nearly 80 prisoners have been in the security housing units, where inmates are confined to their cells for 23 hours a day, for more than two decades in California’s Pelican Bay State Prison, according to the Center for Constitutional Rights. California has been working to reform its policies on segregation by court order. In April 2014, in response to a lawsuit filed by several inmates, a federal judge ruled that California’s practice of placing seriously mentally ill inmates in segregation for extended periods "can and does cause serious psychological harm" and violates the 8th Amendment. He also ordered that inmates should not be held in administrative segregation for longer than 72 hours for non-punitive reasons.

Thomas Silverstein has been held in solitary confinement since he stabbed prison guard Merle Clutts to death at Marion State Penitentiary in 1983. The murder, which Silverstein plotted for months, helped lead to a prison-wide lockdown that would become the foundation for modern high-security facilities. Silverstein has since been transferred to ADX Florence, a federal supermax prison in Colorado, where he is serving life without parole for killing Clutts and two prison inmates years earlier.

Herman Wallace was serving a 50-year sentence for armed robbery when he was convicted, along with fellow inmate Albert Woodfox, for the murder of a prison guard at the Louisiana State Penitentiary in 1972. They maintained their innocence and with another African-American inmate, they became known as the "Angola Three." Wallace spent 41 years in solitary confinement. In 2013 his conviction was overturned on the grounds that he’d been denied a fair trial and was released. He died three days later. Woodfox’s conviction has also been overturned but the government is fighting to reinstate it. Woodfox is now in his 42nd year in solitary.