That would be difficult, Sumo 19, because the maximum penalty for GBH is 5 years imprisonment (unless the offence was racially or religiously aggravated, in which case it's 7 years, or unless it's 'GBH with intent', which is a completely different charge).
Sentences for 'GBH' are shown on page 15 of this document:
http://www.sentencing...inst-the%20person.pdf
Those sentences refer to a first-time offender convicted after a trial. An early guilty plea can see the sentence cut by one third, whereas previous convictions can push the sentencing range higher (but not beyond the maximum stated above).
The exact sentence imposed will depend largely upon the extent of the injuries sustained by the victim, so it's impossible to give a realistic estimate without knowing the full details.
However, it's also possible that the court might pass an 'IPP'. See 'Imprisonment for Public Protection' here:
http://www.hmprisonse...fesentencedprisoners/
If so, a minimum period in prison will be specified by the judge but the offender won't be released from prison until it's deemed safe to do so.
Chris