Does involuntary separation pay reflect on your LES? Also how long does it take to receive?

no idea how long it takes but you get an LES for any pay you receive.

It will appear on your last LES and you will receive it with your last pay. Please note that if you are taking terminal leave it will not be paid out until your last "official" day in the army -- that is to say, the day your leave ends and you are required to hand in your active duty ID card.