A correspondent writes:
How long does one leave the ashes from the Ash Wednesday service on ones forehead?
Is it wrong to wipe them off?
It is not wrong to wipe them off.
As far as how long one leaves them on, there is no standard answer. It is a matter of your choice.
As a matter of public witness, I would recommend that you leave them on until you are done going out in public on Ash Wednesday.
I would defintely wipe them off by the morning of the Thursday following Ash Wednesday.