Ignoring, for the moment, the possibility of an IPP sentence:
The 'starting point' sentence (from which a judge must work up or down) for "pre-meditated wounding or GBH involving the use of a weapon acquired prior to the offence and carried to the scene with specific intent to injure the victim (but not resulting in a life threatening injury or particularly grave injury)" is 8 years imprisonment. Unless there are really exceptional circumstances the judge must pass a sentence in the range of 7 to 10 years.
However, if the victim "suffered life-threatening injury or particularly grave injury" (under the circumstances referred to above), the 'starting point' moves up to 13 years, with a sentencing range available of 10 to 16 years.
Those sentences though refer to a first-time offender, convicted after a trial. Your ex's previous conviction(s) will push those figures higher but his guilty plea (if it was at the earliest possible opportunity) can see the sentence reduced by one third.
So, assuming that the victim's injuries were not 'life threatening or particularly grave', I'd guess at a sentence of around 9 years less one third, i.e. 6 years (of which half would normally be spent 'inside').
If the court decides that the injuries should be regarded as 'life threatening or particularly grave', I'd guess at 15 years less one third, i.e. 10 years (with 5 years actually spent 'inside').
As you mention, there's the possibility of an IPP. There has been some concern from prison reform groups that an IPP prisoner can't be released until he's convinced the authorities that it's safe to do so, yet many prisons don't run the courses which can help an offender towards rehabilitation. Consequently it seems likely that many people who are given an IPP sentence will remain in prison for many years after the end of the tariff term, possibly for life.
Chris