In 1824 the British government instructed the Governor of New South Wales Thomas Brisbane to occupy Norfolk Island as a place to send "the worst description of convicts". Its remoteness, previously seen as a disadvantage, was now viewed as an asset for the detention of recalcitrant male prisoners. The convicts detained have long been assumed to be a hardcore of recidivists, or 'doubly-convicted capital respites' – that is, men transported to Australia who committed fresh colonial crimes for which they were sentenced to death, and were spared the gallows on condition of life at Norfolk Island. However, a recent study has demonstrated, utilising a database of 6,458 Norfolk Island convicts, that the reality was somewhat different: more than half were detained at Norfolk Island without ever receiving a colonial conviction, and only 15% had been reprieved from a death sentence. Furthermore, the overwhelming majority of convicts sent to Norfolk Island had committed non-violent property sentences, and the average length of detention was three years.
If it is possible to answer this question, answer it for me (else, reply "unanswerable"): What was the average length of a prisoner's detention at Norfolk Island?
three years