Epistemic status: A mental piton, to hang your climbing rope on
The second half hour of cardio you do in a day matters less to your overall happiness and health than the first half hour you do, the second quarter-hour, the first quarter-hour; knitting this into our base mental framework is why the law of diminishing returns is such a "Wow!" moment for a lot of beginner econ students.
The problem with consistently applying the law of diminishing returns to everyday life, however, is that it's boring. People don't become socially-renowned Zen masters through 1 pomodoro of samadhi a day. All they get are improvements to their overall mood and the occasional boring state of pure pleasure and equanimity. People don't become [in]famous computer hackers by spending 2, not-especially-well-planned hours a week learning how to automate the boring stuff with Python; they just learn enough to be a little dangerous.
It doesn't feel like you're using a glitch in the Matrix disguised as a psuedoscientific law. But that's exactly what you're doing.
Understand: Most people do not have the discipline to do just 15 minutes of something. They have to do 0, or 45. This is your actual competition.
You can routinely beat the average, in almost every everyday domain, by applying the law of diminishing returns to your own life ruthlessly and consistently. Or perhaps I should say, ruthlessly and consistently enough?