if you got three years in jail

[fasttimes]

how long would you have to do ie how much remission would be allowed for good behaviour

[Royal Pollen]

half i think

[Buenchico]

(2-part post):
It's possible that the rules might have changed recently but the cut-off point used to be 4 years, rather than the 2 years cited by Eddie51. The basic structure was (and, to the best of my knowledge, still is) this:

Sentences up to (but not including) 6 months: Release at half-way point of the sentence. For the remainder of the sentence period, the person is 'on licence' and can be returned to prison if the terms of the licence are broken. There is, however, no supervision by probation services.

Sentences from 6 months, up to (but not including 4 years): The same as above except that from release, through to the three-quarters point of the sentence, the person will be supervised by probation services.

Sentences from 4 years upwards (except life sentences): The prisoner can apply to the parole board for release at the half-way point of the sentence. If this is refused, the prisoner can apply annually thereafter. If the parole board do not authorise release, the prisoner will be released at the two-thirds point of the sentence. Irrespective of the time of release, licence conditions apply to the end of the full sentence period and the person will be supervised by probation services up to the three-quarters point of the sentence.

[Buenchico]

When determining the date for release, the following will be taken into account:
1. Any time spent in custody, prior to conviction, will count as time already served.
2. Any time spent in police custody (including, for example, any date upon which the person had to attend a police station to answer bail), plus days on which the person had to appear before the courts, count as time served.
3. Prisoners are not normally released at weekends. Anyone with an official release date on a Saturday or Sunday will normally be released on the preceeding Friday.

As I've said, the above is, to the best of my knowledge, the current basic structure. Additionally, however, there are early release (tagging) schemes which permit the release of some offenders before the half-way point of the sentence. These do not normally apply to certain types of offences (e.g. sexual offences and serious offences of violence). Where the scheme does apply, some offenders might be released after serving about one third of their sentence.

There are also other variations to the basic scheme which might apply to particular types of offence. (e.g. sentences for some sexual offences may include a longer period of supervision than would normally be associated with that length of sentence).

Chris