What factors affect the speed that instinct goes down when used to go past NPCs dressed in the same clothes?
If there are more NPCs that notice you go past them, will it go down faster?
It appears to be a flat rate cost. When activating it for blending in, it will subtract a certain amount (perhaps more on higher difficulty levels), then slowly burn through it as you continue to use it (again, the burn rate might be faster on higher difficulties). I will assume this happens to prevent people from "pumping" the button in an attempt to only burn instinct when the awareness meter goes up (IE, it forces you to hold the button down and burn instinct until you're clear from everyone). This was tested on Hard mode using instinct to walk through different size groups of people while wearing the same clothes as them.