The standards rules apply.
- For a straightforward summary offence it is 6 months from the date of commission.
Sometimes, under really limited circumstances an application can be made to extend but that is very very rare. - In the case of either way offences - False Records for example - there is no time limit at all.
In terms of Fixed Penalty tickets at the roadside, DVSA are only proceeding with matters that are current at the time of Encounter where the Police - quite correctly IMO - will go back 28 days.
Hope that helps.