I do not know about the program you are discussing here,but according to the little information in “The Allan Wells Book of Sprinting” it seems to me the speedball was present in each of the phases (Gym Fitness,Track Fitness,and Race Fitness) of his program,and possibly varied and periodized week by week, as all other stimuli (exact same daily format for a week,then change).
Personally I would keep such a fast stimulus as the background noise of any program,and do it throughout the year,keeping the very same protocol consistent for extended periods of time (up to 6 weeks and more),only aimimg at ever faster cadences over a same routine,before moving on and change it.