Measuring pulse width with limited components
One resistor and one capacitor, forming a low pass filter, removing everything but the DC component. Unity-gain buffers can optionally be added on the input and output, if necessary to isolate the circuit from external loads.







simulate this circuit &ndash; Schematic created using CircuitLab

With a pulse frequency of 1 kHz and component values selected to give a relatively low ripple, this is the result from a simulation in LTspice showing the DC output at levels 10 mV, 100 mV and 300 mV respectively from an input pulse width of 10 µs, 100 µs and 300 µs. If you have a different pulse frequency (though you did not mention anything about that in the question), you can scale the output by inserting a resistor across C1 as a voltage divider, or add a gain before R1 using an opamp.