What decides the response time parameter requirement for zener diode
Most protection circuits are based upon the energy that is needed to be absorbed and there is no explicit time associated with that.  That is because one is normally designing to protect against ESD transients.

If you know that you are going to have a sustained fault condition then you must design for it as most protection circuits with sustained fault will overheat and fail.

That means that a sustained fault condition is now part of the specification and thus must undergo a full design approach. i.e It must be designed for.