Normally, a convicted person who has received a prison sentence in excess of 12 months will spend half of the sentence in prison, and will be released to spend the other half on licence in the community.
Rape and murder are the two most serious crimes in the book. A murder victim is dead, and a mandatory life sentence applies. On conviction the sentencing judge decides on a minimum term to be served in prison, after which a parole board decides whether or not the prisoner can be released into the community on parole. Around 300 people a year are given a life sentence for murder, and currently there are 5,500 people in prison serving sentences for it. Around 200 are released on parole each year.
If the sentencing judge in a murder case considers that the circumstances of the murder are particularly serious, he or she can impose a 'whole life' sentence which means the convicted person will never be released. Currently, there are 59 people serving whole life sentences.
Most people acknowledge that rape is a particularly vile crime - the victim carries the horror of it for life - and the law recognises the fact by allowing a maximum sentence on conviction of life imprisonment. In practice, the sentencing guidelines mean that convicted rapists spend between 4 and 19 years in prison. Sentence length depends on many factors - the severity of an attack, whether extreme violence accompanied the rape, whether the attack was pre-meditated, etc., etc.
In the case in question, the man was a serial offender, he planned his offences, and used drugs to subdue his intended victims. He was convicted of 19 charges of drugging and sexually assaulting 12 passengers and one charge of rape, but it is known that he carried out many more offences - possibly as many as 500.
On conviction he was given an indeterminate sentence, which meant that it was up to the parole board to decide if and when he was considered to no longer be a danger to women, and released on licence. he has served ten years in prison.
The controversy surrounding his imminent release appears to centre around the fact that his victims were not given prior notice of it, that they were under the impression that he would be in prison for 'a very long time', and that he has not been tried for other offences where the victims had come forward.
I certainly don't know whether or not this man is still a danger to women - none of us do - but I can offer an opinion. My belief is that sex offenders are always going to represent some danger to society. the feelings that drove them to commit their crime or crimes in the first place are not simply going to vanish just because they spend time in prison. The extent of the danger to society when they are released depends entirely on their ability to a) understand the seriousness of what they did and to feel remorse for it and b) to control their desires sufficiently to stop them ever doing the same thing again.