Tom:
given the pulse frequency varies 3:1 ( 2....6 Hz ) and the number of pulses is unknown, the ideal diagram you draw is impossible to acheive.
The best you can do by whatever means is a (near) instant response to the first pulse, and then a delay of at least 1/2 second to detect an end of pulses.
Allan
Allan-
I'm not so sure the frequency DOES vary. I was taking it the OP has a signal somewhere around 2-6Hz, but no definitive answer.
Since this is an add-on to a battery monitor alarm circuit, the alarm surely is active for more than mSecs. Again, more information would help. Same with what the plan is for the signal he seeks.