Subscribe & Save Algo

Does anyone have any ideas to how the subscribe & save algo works?

It seems to me that only one seller can have s&s and once they get it they have it forever. I’ve seen sellers literally not have any stock for over 2 years then suddenly reappear with s&s. I’m currently going after a few s&s but if other sellers who don’t have live offers have s&s then I’m wasting my time.

As for how to get s&s, it looks like holding the buy box for 120 out of 130 days or something similar seems to get works.

Any opinions are would be welcome.