Excellent question. Mary, being without original sin, was justified at conception. Not having sinned once, she remained in the state of contunual justification all her life.
That's a bit odd. Perhaps I've never thought of it before but how could Mary possibly be without original sin as the sin nature is past through the father? That would mean that Mary would had to have been born of a virgin birth, like our Lord. Is that what is taught?
On another note, what you are saying is that Mary was not justified by works or faith. In fact, it appear what you are saying is that God justified Mary. Strange that He doesn't do this for all. ;O)
No, Mary (according to your doctrine) was sinless and didn't need to be Justified (which is having your sins paid for and made holy).