anywhere from a few month, to 18 months, starting from when you cease causing inflammation to the skin
And sometimes longer. I have a PIH spot from an accident with my inline skates close to my left elbow. This occured more than three years ago, and the PIH spot is just fading away (meanwhile only visible if You know its place).
During the recent past i had to take oral cortison against a sudden and massive breakout of Neurodermitis. That also helped with the hyperpigmentation. But to me PIH is never ever any reason to take drugs i might need in the future agains something really serious.
A slight addition to the causes of PIH: even new hairs breaking through the skin can cause PIH, and it can also occur immediately outside of an epilated spot. I see this from time to time when i do fine hair on the upper lip (but one time also in the throat area during a beard removal): hyperpigmentation just outside the area i actually did.