'Section 18' (GBH with intent) is an extremely serious charge. It's akin to 'attempted murder' and carries the same maximum penalty of life imprisonment.
Unless there are very exceptional circumstances (which the judge must explain in open court), judges are obliged to pass sentence based upon the table on page 13 of this document:
http://www.sentencing...person_accessible.pdf
It should be noted that the sentences shown there refer to a first-time offender who is convicted after a trial. Previous convictions (particularly for violence) can push the sentencing range higher. A guilty plea at the earliest opportunity can see a sentence cut by one third. Only half of the nominal sentence is normally spent 'inside'.
'Section 18' is an 'indictable only' offence, which means that it must be dealt with by a Crown court. It usually takes between 9 and 15 months, from the date of the offence, before sentence is passed. (There will be several court hearings before that).
Your boyfriend's legal team might be able to get the charge dropped to 'ABH' because a simple cut does not constitute 'GBH'. (It would need to be the type of injury which either requires a transfusion or else leads to permanent disfigurement to fall into that category). If your boyfriend is convicted of 'Section 18' he will almost certainly get a prison sentence (of at least 2½ years).
Chris