The 'starting point' for sentencing an offender convicted of 'malicious wounding with intent to do grievous bodily harm' (under the circumstances you've described) is a custodial sentence of 5 years. The judge can move the sentence up as high as 6 years, or down as low as 4 years, depending upon the circumstances.
However those sentences refer to a first-time offender convicted after a trial. An early guilty plea can see the sentence cut by one third. Previous convictions (particularly for violence) can see the sentencing range pushed higher.
The actual time spent 'inside' is usually half of the nominal sentence.
The attacker's barrister might try to argue that the charge should be reduced to 'ABH'. To see the differences between 'ABH', 'GBH or Wounding' and 'GBH or Wounding with intent', read paragraphs 45 to 67 here:
http://www.cps.gov.uk...he_person/#P189_14382
To read the full sentencing tables for each of those offences, see pages 13, 15 and 17 here:
http://www.sentencing...inst-the%20person.pdf
Chris