DWP has announced:

National implementation of the benefit cap will be managed over a 10 week period split into two tranches:

• Tranche 1 will include all local authorities with 275 households or fewer and capping will commence from 15 July 2013;
• Tranche 2 will include all local authorities with 276 or more households to be capped and is likely to commence from the week of 12 August 2013. It is important to recognise that there will need to be flexibility with this date as the clearance of Tranche 1 will affect the precise start date of Tranche 2.

Learning from phased rollout, local authorities will be sent low volumes of cases for capping at the start of national implementation to ensure DWP levels of expertise are achieved at the required accuracy levels. 

Source: http://www.dwp.gov.uk/local-authority-staff/benefit-cap/