I believe that the dive reflex, once strongly evoked, persists for many hours at least.
I have as a tentative hypothesis, for example, that the best way to warm up for a static apnea attempt is to evoke a strong dive reflex by effortless ( to the extent possible) exposure to apnea and hydrostatic pressure, say, by a series of relaxed 1 - 2 minute pulldowns to 30m, followed by a 2 - 3 hour rest (perhaps while riding to static attempt site).
Using myself as a highly subjective subject, I've observed that my static performance, in terms of time and comfort, has been much better than I had any right to expect when this program was followed.
It makes sense that the dive reflex, or the predisposition to it, would persist. Physiology is inherently conservative. It is a homeostatist, a status-quo preserver. That's how living things differ from rocks: they respond, and do so in the service of preserving a prioritized set of states. The dive reflex is associated with preserving the highest -level states in the hierarchy: brain functions, etc., and so it stands to reason that when the environment evokes it, it would remain 'on alert' , as it were, for a some time. It takes asphyxiation very seriously, and it makes sense to expend whatever costs are associate with this alert if the threat of sudden death by asphyxiation / drowning has been detected.