I am afraid I am not in a position to advise you of an exact start date. Frustratingly, HMRC have not finalised the full migration timetable but they have announced plans to have most employers operating Real Time Information (RTI) by April 2013.
You should by now have received a letter from HMRC telling you what you need to do in the run-up to RTI. And HMRC are planning to write to everyone again in February 2013 with confirmation of their start dates.
If you would assistance in ensuring your payroll is RTI compliant, please feel free to contact your local TaxAssist Accountant.