To know what value of capacitor and resistor you want, you will need to think about the response speed you need to achieve. Looking at the waveform, it looks like you get a pulse every few seconds, and the pulses last tens of milliseconds. This is pretty slow, so:
- The analog signal will be much slower than your pulse rate. You should expect the analog signal to take about a minute to settle when the pulse rate changes.
- You will need to choose pretty large values of resistor and capacitor. The time constant of an RC filter in seconds is the product of the resistance in Ohms and the capacitance in Farads.
Since you're looking for something in the order of a minute, I'd start with 12k ohms, and 4700uF. See how that works. If you need a slower response, then use a larger resistor.