Denver - The time has come for jurors to hear whether James Holmes should be executed for killing 12 people in a Colorado movie theatre. But even if they decide on death, Holmes could spend the rest of his life in prison awaiting capital punishment that never happens.Colorado has executed only one person in nearly half a century, and just three people sit on the state's death row. The man closest to seeing his death sentence carried out was granted an indefinite reprieve in 2013 by the state's Democratic governor, who said he had doubts about the fairness of the state's death penalty system."Capital punishment is on life support in Colorado," said Denver defence attorney Craig Silverman.As a prosecutor, Silverman secured a death penalty verdict against a man for kidnapping and killing a woman in 1984. Sixteen years later, Frank Rodriguez died on death row from Hepatitis C complications.The same jurors who convicted Holmes of 165 counts of murder, attempted murder and other charges in his July 20 2012 theatre attack must soon decide whether he should pay with his life. The sentencing phase of his trial begins on Wednesday.The district attorney who prosecuted Holmes, George Brauchler, said that if any crime should be punished by death, it is this one: He opened fire on an audience of more than 400 defenceless strangers in a darkened theatre during a Batman movie premiere, killing 12, wounding 58 and leaving 12 others injured in the mayhem he caused.But many obstacles stand between Holmes and execution.Death row inmates in almost every state spend decades in prison as mandatory appeals play out in court. But Colorado has adopted a unique system for death penalty appeals, requiring those sentenced to death to file post-conviction claims before a higher court reviews their case. It was supposed to speed up the process, but "it actually slowed it down exponentially", said Hollis Whitson, a Denver defence attorney who specialises in appellate law.Holmes' appeals could be even more complex because of his mental illness.Doctors testified he suffers from schizophrenia. If his mental state deteriorates while he is on death row he may never be executed, said Michael Radelet, a sociology professor at the University of Colorado, Boulder, who has long studied the death penalty and opposes it."If he is sent to death row, we're going to need dump trucks full of money to pay the mental health experts who will continue to argue this for the next 20 years," Radelet said. "Even if Holmes is sane today, there will be inevitable questions about his sanity at the time of execution."