At what point would you step up to ferrea hollow stem valves

RPM isn’t nearly as important as overall valve length and head diameter. There is no arbitrary RPM where you switch from stainless to titanium valves. Or even when you switch to smaller stems. There is never a negative going to a lighter valve.
I don't disagree with this post, but it does seem to me that valve length is partly related to rpm from the perspective that a longer valve allows for a taller installed height for the springs which are required for higher rpm with a given cam lift. You can't get one without the other, whether it is mechanical clearance to the guide for high valve lift, or adequate spring pressure to control the valve train with that lift a a certain rpm. They are all interrelated imho.