The operator will present his request to the local CAA with a scenario. It could be a multiple of distance or; the first hour is a bit longer because you initialy have a better TAS (you are descending from a higher altitude) and then you add multiple from the distance at the driftdown altitude(fix). All these are made with an assumed average weight (consistent with your ops) and a speed schedule of your choice (there has to be 3 or 4 options in your FCOM). Once this has been approved by your CAA, those numbers will be used to create your areas of operation (60,90,120,138,150,180,207 min) and inserted in your company docs (it is actualy an ops spec granted to you by your CAA).