Is There a Minimum Time Required for Adaptation?

Exercising in zone one has an effect on the level of adaptation. It would be because you're recruiting more muscle at a higher frequency, and have a greater metabolic flux. But what we're trying to do is stimulate those adaptations without triggering a big stress response. So if i go out for 20 minutes an anzon one, does that really do anything? Or do i need to hit like, 90 s before i get any advantage? I think 20 minutes sounds like almost a waste of time.