How to calculate the rotation time of the hall flow meter sensor?

Question

Arduino

Help, I can't figure it out.

A similar flow meter detects pulses on the principle of a Hall sensor. X = pulse frequency Hz.
I found how to calculate the flow rate of water.

This is the Pulse Frequency /7.5
And we will know the water flow from this pulse frequency.

And how to find out the number of milliseconds of rotation of this very impeller?

Answer 1

Impulses went - they began to count the time. Run out of impulses - finished counting.

Answer 2

And how to find out the number of milliseconds of rotation of this very impeller?

There is a limit at zero flow and speed, so you have to set some finite time interval for the decision "there is no more flow." The rotation time will be "from the first to the last pulse up to the selected decision interval".
Or - do you mean the case "there is no more flow, but the impeller rotates by inertia"? I think this doesn't happen. Because the impeller is light (to minimize counting error) and the viscosity of the water is high enough to prevent this.

And if, theoretically, the impeller starts to spin in reverse,

The same, itself - will not be able, only when the flow is reversed.