The DWP has produced a detailed table setting out the staging dates for automatic enrolment.

The requirement for employers to automatically enrol eligible jobholders into a qualifying pension scheme and pay contributions will be introduced in stages from 1 October 2012 to 1 September 2016, starting with the largest employers.  

The staging dates may, of course, change if we have a change of government: the Conservatives have indicated that if they win the election they will implement the automatic enrolment regime but may shorten the timetable.