How long do you have to press charges against common assault in UK?

6 months on summary offences. There are no limitations on proceedings for indictable or either way offences. So first you'd need to work out which it was. If you had any kind of visible bruising however long it lasted it would be an 'assault occasioning actual bodily harm', which is triable either way. However even then you wouldn't get anywhere as at this stage it would not be in the public interest to run your case. I.e. CPS would be so sure that you would not get a guilty judgement, that it would not be worth wasting public money to take the offender to court. If there were no visible injuries at the time it would indeed be a common assault, and therefore too long ago to deal with.