How long does an isotretinoin capsule take to start effecting your skin

I'm not asking how long it takes for Accutane to start working, I know it's about 2 weeks to a month; I'm asking how long after taking a SINGLE pill does that one specific pill enter your system and do its stuff etc.

I'm asking because I want to time shaving my beard when my skin is less sensitive. I know the sensitivity is there for the entire course but, if for example I take the pill early morning, at what point of the day is my skin the most sensitive? Right after taking it or some hours later?