In CrossFit one of our primary goals is to maximize power output. A well known CrossFit axiom is that power output is equal to intensity. In order to maximize power output we typically need to scale the workout. The question is how to scale a workout so that one maximizes power output and also trains the body to adapt to the speed in which the workout should be done.
I believe that on most 21-15-9 workouts one should attempt to perform them as close to unbroken as possible. For instance Fran (thrusters, pull-ups) is often done by elite CrossFitters in under 3 minutes. At 3 minutes that is just under 2 seconds per rep.
I ascertain my scaling by selecting the maximum amount of load I can do 21 unbroken repetitions of each of the movements at. When performing Fran I would select the maximum weight that I can execute 21 unbroken thrusters. For pull-ups I would either increase my weight with a vest or decrease my weight with bands so that I could execute 21 unbroken pull-ups. For some athletes this could mean going with RX for each movement and for others it could mean increasing or decreasing the load. The athletes unique strengths and weaknesses will dictate how to scale.
I find using this formula my 21-15-9 workouts typically take about 3-5 minutes. I feel that is the optimal time to perform this type of workout in. The only time I would scale the WOD to longer than 5 minutes is with deconditioned athletes that are not ready to perform the workout at that speed. The only reason to scale so that one can go faster than 3 minutes would be to set a PR at the RX.