HIE actually does address this, but not explicitly. HIE says cardio duration should not increase, rather intensity should, with increasing fitness - and that the total workout session should not exceed one hour. Also, HIE/NHE prescribes interval training which confounds the time issue because it consists of interspersed periods of high-intensity activity and lesser-intensity or no activity. Using the example of hitting a heavy bag, it would be a lot to ask of anyone to perform intense bagwork for one-hour even with brief intermittent rest periods. So the bottom line is in terms of “optimal exercise” (as opposed to optimal enjoyment or optimal sports performance) it's unproductively exhausting to perform high-intensity interval training for more than 45 “total minutes” per session. If you’re doing lower intensity continuous exercise, like jogging, one-hour would be the cut-off. (This does not mean someone who runs for two hours at 8-minutes per mile can’t derive any cardiovascular benefit beyond that obtained by running for one hour at 8-minutes per mile. It does mean that running faster and stopping at one-hour is superior hormonally to running slower and longer.)