Racing roadkill
Guru
Give yourself 48 hours clean and clear after any ride where your heart rate was elevated for any significant length of time ( that's personal to you) there certainly is no 'one size fits all' for the criteria here. But you will have a 'performance curve' where time spent at a certain intensity of exertion and time spent doing it, plotted against performance gain ( measureable by FTP, amongst other metrics) will follow a certain pattern. If you operate too far above that theoretical curve, for too long, you will actually end up worsening your fitness level, not improving it. You have to operate above the curve to improve rather than maintain your condition, but the trick is to know how far above your current 'curve' to go, over time, in order to improve, rather than knacker yourself.