'Common assault' is a normally 'summary only' offence (which can only be dealt with by a Magistrates Court, rather than a Crown Court). As such, the 'laying of information' must occur within 6 months of the offence.
More serious types of assault (such as 'ABH', GBH' or 'GBH with intent') are 'either way' or 'indictable only' offences, where there is no time limit for prosecution. (You could still be prosecuted half a century, or more, after the offence).
Chris