With the obvious exception of "overtraining" via tearing something while stretching far beyond your limits, could you theoretically stretch all day and keep getting more and more flexible throughout the day? Muscle protein synthesis has a limit to its rate (you can only repair muscle damage so fast*), but part of stretching is downregulating the neural pathways that lead to your muscle contracting to resist the stretch. I'm not aware of any limit on the rate of neural adaptations since it's purely mental in a sense. Therefore, shouldn't there be no ceiling, or at least a much higher ceiling to the amount of effective stretching "volume" you can do before reaching diminishing returns? Anecdotally, I've stayed up the whole night stretching before (mainly on my phone in uncomfortably stretched positions) and become more flexible hour by hour. * I appear to have read somewhere that the muscle damage --> repair theory of hypertrophy is dodgy for some reason, but I don't remember how credible it was.