Does anyone have a specific Nursing policy on the time frame they have to respond to a low electrolyte lab (K, Phos, Mg)?

One of the issues we have with our electrolyte (K,Mg,Phos) protocol orders is that the RNs do not act on the lab value in a timely manner. The question has come up, what exactly is a time window in which they can still act on an "old" lab value? Do you have a policy defining when the level is too "old" to act upon, do you recommend rechecking the level or getting a Physician order to proceed per the protocol despite the "age" of the level?