To add some more science, as of now, the “shortest time” is the Plank time which is the time it takes light to travel one Plank distance. It’s unimaginably shorter than a femtosecond. Of course Plank time would be the upper limit as far as science is concerned. If dhammā are all ruled by physical law (the broader definition of “physical laws”) then that would be the speed of cittā arising.
However, as far as “matter” is concerned, the shortest lifetime of an elementary particle would be the lifetime of certain bosons which is in the yoctosecond scale, which is a septillionth of a second (a quadrillion billionth of a second).