Is there any research on how quickly responses like this decay (e.g. over generations) once the conditions that supported them no longer obtain? Some casual Googling got me nowhere, and I'm curious.

As far as I know there isn't research on humans about such significant traits. Especially not the highly unnatural case where the self sustaining momentum aspect is also removed. (If there was merely a change in environment then we would expect the adaptation to take longer because sexual selection for the sake of nothing more than more sexual selection of descendants.)

I know there have been studies on various creatures in labs and observation of the rate of adaptation of traits in wild populations of less-than-human animals. I have little idea how much information that can give us about adaptations in humans and don't know to what extent human changes have been analyzed.