Common assault is a 'summary only' offence, which means that it can only be dealt with before a magistrates' court, not a Crown court. (There are exceptions to that rule but, unless your ex was also facing charges 'on indictment', they don't apply here).
So the maximum penalty he might face is 6 months imprisonment. Given that he has previous convictions and pleaded not guilty, it's likely that he'll face a custodial sentence somewhere between 3 and 6 months. (He'll actually be 'inside' for half of that period).
That assumes that his previous convictions include violent offences and that you suffered some form of visible injury (e.g. grazes or bruises). If his previous convictions were all for,say, shoplifting and you suffered no visible injuries he might just receive a non-custodial sentence, such as 150 hours unpaid work.
Chris