Left Side is ordinary 555 monostable timer,and Right Side is the output pulse width time measuring circuit.
Output pulse is converted to 1V-pulse with zener diode and 1uA/V voltage current convertor is charging the 1uF capacitor .
The voltage of capacitor is directly proportional to the time(seconds) of pulse width.1V=1sec.(v=1/c[idt)
5-pin of 555 is voltage control pin.Usually the voltage of 5-pin is fixed 2/3*Vcc(in this circuit it is 3.33V). By changing this 5-pin voltage we can change the monostable timer time.
In R=1Mohm and C=10uF we can calculate the pulse width=1.1RC(11sec). When the switch of 5-pin is OFF, time measuring capacitor's voltage will show 11.2V(11.2s).
When the switch is ON ,5-pin is applied the outside voltage. For example we apply 4.9V to 5-pin, the timer's time will extends to 76.7s(76.7V).
Change the control voltage from 1V to 4.9V, then you can see thećtime extention. (Don't forget the reset of the charge of 1uF)