He has been diagnosed with schizophrenia, according to his lawyer Johannes Hadi.
The court heard how Duc had recalled hearing a voice in his head telling him there was a bomb on the plane and to shout that out to alert others.
The plane had to be escorted to Singapore by fighter jets.
"His delusionary beliefs, auditory hallucinations, as well as psychotic thought disorder have greatly contributed to his offending behaviour," the judge said during the sentence hearing.
The jail term was backdated to his arrest and Duc would be released on Friday and return to the United States.
($1 = 1.4131 Singapore dollars)
