Just saw this. I don't have a definitive answer, but I would imagine it depends on how much you need to improve your personal flexibility, and how much the workout is helping you do that. I'm 63 years old and have lost quite a bit of flexibility in the last few years. I was never super flexible to begin with, though. Right now, I'm doing this workout once a week, and I see improvement each time, if that helps.
With flexibility/mobility I find once a week is enough to maintain my current level of flexibility but I don't see an improvement. Twice a week I will see a bit of improvement after a while. To see real improvement I need at least 3 times and preferably 4 times per week and I will see noticeable improvements within 1-2 weeks. However, if I stop I very quickly lose the gains I've made. What I try to do is put into my workout rotations 2-3 weeks where my main focus is on flexibility/mobility and I will do at least 4 workouts a week of this type, then I will drop it back to once or twice a week to maintain.