The case automatically goes to the Crown court, because it's 'indictable only'.
'GBH with intent' is a very serious charge, as it's only one step below 'attempted murder'. Indeed, since it carries the same maximum penalty of life imprisonment, it's effectively the same charge.
Judges are given guideline 'starting points' when sentencing various offences. They're also told the range which their actual sentence should normally fall into. A 'basic' GBH+I offence carries a starting point of 4 years imprisonment, with the sentencing range being 3 to 5 years. However, if your partner's injuries are regarded as 'very serious' (which they may well be), the starting point goes up to 5 years custody, with a range of 4 to 6 years.
However, those guideline sentences apply to first-time offenders convicted
after a trial. An early guilty plea would result in a reduction of the sentence.
It's hard to be accurate without knowing all of the circumstances. My best guess is that the court would opt for the lower end of the 4 to 6 year range (i.e. 4 years) and then reduce that down to
3 years for an early not guilty plea. That would normally mean that you'd spend 18 months 'actually inside'.
Reference:
(This is the document which the judge will have before him when deciding upon the appropriate sentence):
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
Chris