It might be a few months after committal before the case actually reaches the Crown Court. This will be a 'pleas and directions' hearing (although there might be one or two administrative hearings before this). If your boyfriend pleads 'not guilty' a trial date will be set (or, at least, the initial stages of getting a trial under way will be started). If he pleads 'guilty', a date will be set for sentencing. Once again, there may be a delay of several months before anything actually happens.
GBH with intent is regarded by the courts as roughly akin to 'attempted murder' and carries the same maximum penalty (life imprisonment). Even for a first time offender, it's extremely unlikely that a non-custodial sentence would be passed. You've not provided details of the actual assault but the minimum range of sentences for a first time offender convicted after a trial is 3 to 5 years imprisonment. However, your post suggests an element of pre-meditation which increases the range to 4 to 6 years. (In the most serious cases, the guideline range is 10 to 16 years).
If your boyfriend pleads guilty, there will be a reduction in the sentence but I'd still expect (depending upon the severity of the assault) a sentence of at least 2� years, with somewhere around 4 or 5 years being more likely.
You can read the guidelines that the judges will use, here:
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
(See the table on page 13 of that document).
Chris