There is also a weird, robust mortality relationship where book readers live longer than periodical readers "regardless of gender, health, wealth, or education" [1]. In those studies, the threshold was 30 minutes of book reading a day.

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC5105607/

While this is just a rule of thumb, I consider moderate exercise, reading, and socializing to be roughly equivalent in positive health benefits. I try to get a little bit of each every day, and then try to have longer sessions of each once or twice a week.