I think that Ratter15 is being rather optimistic. The vast majority of offences of 'GBH with intent' result in a custodial sentence. Where someone has previously been convicted of GBH, the likelihood of a custodial sentence effectively becomes a certainty.
The most recent figures I can find for the lengths of sentences are for 1993. In that year, the shortest custodial sentence passed was 6 months. The longest was life imprisonment. The average term was 4 years 2 months.
http://www.publications.parliament.uk/pa/cm200 506/cmhansrd/cm050712/text/50712w19.htm
The courts always regard 'glassing' as a serious offence. In view of your boyfriend's previous convictions, I'd guess at a sentence of 4 years.
Chris