Permanent marker will stay on skin for around 3 days before it starts to fade away.
When writing on your skin with permanent marker the permanent marker will eventually come off of the skin overtime.
Even if you do not remove the permanent marker from your skin the permanent marker will fade and completely disappear from the skin.
However you can use some rubbing alcohol or acetone based nail polish remover to get the permanent marker off the skin.
It's not recommended to write on your skin with permanent marker but it's not really gonna harm you if you do.
I've written on my skin with permanent marker for years and never gotten sick from it.
But it's not recommended to do so.