Spanning 1982 to 1992, the offending occurred when the victim was aged about 3 to 13 years.
It had had a profound effect on every part of the girl's life, the ruling said.
The abuse stopped only when, aged about 14, she told her mother.
A complaint was laid with the police in 2008 when the man was arrested for indecently assaulting another girl.
The accused suffered a severe hearing disability and health problems resulting from heavy drug and alcohol consumption.
The Court of Appeal noted that if sentencing had immediately followed the offending (before September 1, 1993), under the Criminal Justice Act 1985 the appellant would not have been eligible for parole. But he could have qualified for remission of sentence after he had served two-thirds of his jail term.
Because minimum periods of imprisonment did not exist then, the court would have had no power to impose one.
The appeal was allowed, reducing the total sentence to eight years and doing away with the minimum five-year period the man must serve.