Sean McKay, 43, was jailed for life in 2008 after subjecting the terrified woman to a horrifying 13-hour ordeal.
At the time he was ordered to spend at least nine years behind bars, but that was cut to six years by Appeal Court judges yesterday.
During his trial, the court heard how schizophrenic McKay held his 27-year-old victim prisoner at his flat in Abbeyhill on January 23, 2008.
Armed police were called and cordoned off the area overnight.
The court had heard that McKay met the woman in a cafe near his home. He lured her back to the property after claiming he wanted to buy art from her. Once she was in the flat, he produced a large kitchen knife. Over the course of the next 13 hours the woman was raped repeatedly and indecently assaulted.
At the time of sentencing, judge Lord Kinclaven handed McKay an Order for Lifelong Restriction which meant he would only be freed from prison when parole board bosses decided he was no longer a threat to public safety.
Lord Kinclaven ruled that McKay should only be considered for release after he had served nine years. McKay thought the term was excessive and his legal team argued that, after a recent Appeal Court judgement, the way in which judges had calculated the length of his minimum sentence was incorrect.
At the Appeal Court in Edinburgh yesterday, Lord Mackay of Drumadoon and Lord Brailsford agreed with the arguments.
However, in a judgement issued by Lord Mackay, he warned the rapist would not be automatically released after serving six years.
He said that it was still possible for McKay, who has two previous convictions for sexually assaulting teenage boys, to spend the rest of his life behind bars if he was still deemed to be at a high risk of reoffending.
The judge wrote: “In allowing the appeal and fixing the punishment part for six years, we stress that it is the minimum period that the appellant will require to serve in custody before the parole board can consider whether and if so when the appellant should be released back into the community.”