Scott Wyatt, 32, handed himself into police earlier this year and confessed to the two counts of arson.

The man was a suspect at the time, but he denied the acts in a police interview and was never prosecuted.

In June, a judge granted Wyatt bail to go to the Karralika rehabilitation program.

But Wyatt did not "settle into a routine" at Karralika, Associate Justice David Robinson said, and his mental condition deteriorated.

He was involved in an assault or threat to another patient. He left the facility and when he returned screened positive to cannabis.

Hospital notes said he left Karralika again and reported to be on a "massive drug binge" and was homeless.

In July he turned himself into the ACT Supreme Court.

He was "glad to be back in custody".

Associate Justice Robinson said Wyatt's mental condition had deteriorated and he was unable to function outside the environment of a prison.

He said the man's long-term prospects of returning meaningfully to community life were "difficult to judge" at this stage.

Authorities at the AMC were reconsidering the man's medication, the judge said.

On Thursday, Wyatt, also referred to as Scott Hutchinson in court documents, returned to the ACT Supreme Court to be sentenced for the two counts of arson.

Associate Justice Robinson said he would imprison the man for two years and six months and make him eligible for parole in six months.

He said he would leave it with the parole authorities to determine when Wyatt was ready for release.

Wyatt's sentence expires in June.