Hi Porpoise, I think it makes no sense for the mindfulness to be part of akusala citta, or unwholesome mind.
I think that maybe the difficulty here lies in trying to view these as discrete moments, which are separate from each other... this can cause the problem where it seems like the mindfulness won't have the ability to be in touch with of what is akusala citta, or even to arise out of it. That is only a conventional description... not the reality.
If we try to describe it as a flux, where things are blended together, then I think we also still have the same problem... i.e., the idea that mindfulness could be a part of akusala citta. That obviously isn't the case.
The duration would be another story. I think that with many practitioners, myself included, the mindfulness (if it was that in the first place) will disappear rather quickly... and then it will pop up again sometimes later (which can happen very soon, or after a while), only to disappear fairly quickly again.
Also, I think that maybe what some people call mindfulness is actually concentration.