After writing on your skin with permanent marker or Sharpie marker the permanent marker should start to fade off your skin within 2 to 3 days and after a week the permanent marker should be fully faded off your skin.
When washing in the shower the permanent marker should go away faster but it can still take a few days of showering to get the permanent marker off your skin.
I shower everyday anyway so the permanent marker for me begins to fade within the first day but the permanent marker eventually goes away fully from the skin within a week.
Using some hand sanitizer or rubbing alcohol or even acetone based nail polish remover can help remove the permanent marker from your skin.
Just soak a cotton ball in some nail polish remover or acetone based nail polish remover and rub the permanent marker and it should come off.
The hand sanitizer when rubbed on the permanent marker on the skin can help remove it as well.