What I'd like to more accurately identify is the point at which we get diminishing returns to health from strenuous exercise, in measurable form. Exercise of course provides enormous health benefits, but progressive overload for instance demands ever increasing caloric intake which runs counter to health recommendations. This is probably easier to track than high intensity cardio except in terms of pure caloric need. I deadlift twice my bodyweight at up to 5 reps and consume somewhere in the range of 2000-2500 calories a day. But is reaching the 1000 pound club worth it from a health standpoint? I doubt it, but the evidence surrounding this is sorely lacking.
It would vary depending on type of exercise. Giving my opinions based on being very interested in the subject for years, without bothering with citations.
For muscle-mass, the health benefits come from having higher glycogen storage (helping to prevent diabetes), as well as maintaining function late into life. The latter is subject to diminishing returns, and maintenance is more important than the amount when you peaked. Also, having too much muscle mass would put a strain on the heart.
Daily light movement is important for circulation. 10-12k steps seems to about ideal.
15-20 minutes of moderate-intensity (120-140 bpm) or 4 total minutes of high-intensity (> 160 bpm) cardio improves heart function, blood-flow, and mitochondrial density. Any more seems to be subject to diminishing returns.
Stretching around 10 minutes per day may help prevent cancer and preserve function.
This leads me to the following recommendations:
1-3 weekly strength training sessions of around 45 minutes
This closely approximates what I've gunned for. I'm not aiming to be a maximalist, but want my investments to be worthwhile. My intuition has suggested to me that with cardio (moderate/intense), a little goes a long way and long sessions can eventually yield injury. You mentioned not bothering with citations but if you recall any sources I'd check those out.
You didn't mention it, but the other benefit of strength training is higher testosterone levels.
