Hey there
To be honest I would say as soon as possible. Cambridge is a tool, a very good one but still a tool. The sooner you let go the better in my opinion. Use it to get you where you want to be and then focus on maintenance.
Personally I moved up the plans when I was between 1.5 and 2 stone above my goal weight, staying at 1200 for the bulk of that time to lose the remainder. When I was at the weight I wanted to reach I moved up to 1500, did a couple of weeks to stabilise and then stopped CD.
x