If the charge is simply 'GBH', the maximum sentence is normally 5 years imprisonment (plus an unlimited fine). However, please see the information about 'IPPs' below
If the charge is 'GBH with intent', the maximum sentence is life imprisonment.
For a 'simple' GBH offence, but with a history of convictions for violence, your ex is likely to be imprisoned for 4 to 5 years.
The sentence for a 'GBH with intent' charge is harder to estimate, without detailed information about the nature of the injuries sustained and the way in which they were inflicted. If the injuries only just reached the 'GBH' level, and the attack was neither sustained nor racially aggravated, (but with the past record of violence), a sentence of around 6 years imprisonment might be passed. However, there are circumstances (such as taking a weapon for a pre-meditated attack or inflicting life-threatening injuries) which could push the sentence closer to (or even above) 10 years.
Lastly, when an offender has a history of committing serious offences (and is thus regarded as a danger to the public) a court may pass a sentence 'in the interests of public protection'. This is similar to a life sentence and can be passed under circumstances (eg 'simple' GBH) when a life sentence doesn't normally apply. Like a life sentence, it carries a 'tariff'. For example, if your ex was given an IPP with a 5 year tariff, he'd be kept in prison for a minimum of 5 years. Thereafter he could apply for release but this would only be granted if the authorities were convinced that he was no longer a danger to the public. He could be kept in prison for much longer, or even (theoretically at least) for the rest of his life.
Chris