There is not a minimum as such. Much would depend on the various circumstances because every case is obviously different. All relevant factors would be taken into consideration before sentence was passed.
If a person is sentenced at Crown Court the maximum sentence for S47 ABH is five years.
If dealt with at Magistrates Court the max is 6 months or a fine.
It's more than possible that a non-custodial sentence, as indicated, could be passed.
Assuming that the person charged has access to legal advice, then consult the Solicitor or whoever is dealing with the case for further advice.
Hope this is of help.