Summary offences (those which can only be dealt with in the Magistrates' court) have an absolute time limit of six months to "lay the information" before the court (that is, to start the proceedings). Common Assault is a summary offence.
After this time no prosecution can be launched.
Even within this time, a successful prosecution is less likely as time goes on. In particular, the person against whom the allegation is made is entitled to be warned of a possible prosecution as quickly as possible (in motoring offences this is deemed to be 14 days unless exceptional circumstances prevail). If no warning is given and proceedings are unduly delayed, he can offer the defence that he had forgotten the details and cannot therefore fairly offer his version of events.
In short, justice delayed is justice denied.