See pages 11 to 14 here:
https://www.sentencingcouncil.org.uk/wp-content/uploads/Assault_definitive_guideline_-_Crown_Court.pdf
If the court accepts that tinnitus (which can be a life-long debilitating condition) has resulted from the assault then 'harm' might well be seen as 'greater'. (As it was a sustained assault that also makes it likely that 'harm' will be seen as 'greater').
With a bit of luck though it's possible that 'culpability' might be seen as 'lower'. That would make it a 'Category 2' offence, with a 'starting point' sentence of 6 months imprisonment. However that could be reduced to 4 months for an early guilty plea. The maximum sentence for a 'Category 2' offence is 51 weeks imprisonment (reduced to 34 weeks for an early guilty plea), with the minimum sentence being non-custodial. However I can't see much in what you've written to suggest that the court would move very far from the 'starting point' of 4 months imprisonment (assuming an early guilty plea).
It would then be necessary to consider whether the court would be minded to suspend such a sentence. While I'd regard that as a possibility it would be unwise to assume that it will happen. (Pack a bag!)
So my best guess is a sentence of 4 months imprisonment,
possibly suspended for 2 years. (If the court did decide that an immediate custodial sentence was the only available option then the actual time to be spent 'banged up' would only be half of the nominal sentence).