The Nr decelerationspeed depends on various direct and nondirect factors. Due to the constructiontype, weights of dynamic components, blade profil with specific characteristics,.... there are significant "fixed" differencies between the helicopters.
Flight conditions are responsible for the next input. On a climb Nr will decrease faster than on descent. With an airspeed in the the upper range of the approved speed limits Nr will be decrease faster or on some helicopters it's not possible on higher airspeeds to reestablish a 100% Nr (on steady AR, without flare). A/C weight is important...
Therefore it's not possible to determine the one and only special time to an uncontrollable Nr or uncontrollable flight. And what's the limiting factors? Bladebending with cracks, blade stall, airframe strikes, .... ??? There is a time window, that depends on the named factors.
The publishing of such times is in my opinion to alert the pilots and to give also the unexperienced jocks an impression.
It's not necessary to press water out of the pitch. "Dead man" curve is important and to feel the situation. I'm on safe conditions, unsafe or really dangerous flight conditions.