Just like a tattoo on your body, SMPs don't
fade over time.
However, SMPs are very small dots that are
meant to look like hair, so even a slight fading over time can give the
impression that they have been removed.
Generally speaking, you won't notice a lot
of fading for about four to five years, but it can give an impression of
fading.
However, it's not a big deal because it
will look natural as it fades.
If you want to maintain
a darker SMP, some people choose to have additional treatments over time.