Much of the plot of Episodes V and VI revolve around the idea that Vader's murder of Anakin was only true "from a certain point of view", and it wasn't Anakin's point of view. Although he spent a long time immersed deeply in the Vader identity, he considered that to be a new phase of his life, not a new life completely.
Thus, if we're to examine this question, it's Obi-Wan's judgment we need to consider, rather than Anakin's, because Obi-wan is the one who considers Anakin to have been murdered. But Obi-Wan's estimate of Anakin's time of death is rather slippery. He likely would have considered it to be whenever it was that Anakin had finally fully embraced the Dark Side of the Force, but he had no way of knowing exactly when it was.
But much of the end of Episode III deals with Obi-Wan struggling to accept his pupil's "death", and so this moment is one we can pinpoint with some precision. Even after watching security footage of Anakin slaughtering the Jedi Younglings, Obi-Wan cannot bring himself to battle his student. Even as they begin to fight on Mustafar, he still tries to coax his student back to the light. He goes so far as to blame himself for failing to train Anakin properly. This continues up until the moment when Anakin disowns his heritage ("From my point of view, the Jedi are evil!")
Then, and only then, does Obi-Wan declare Anakin "lost". This is where he begins to see his student as gone. As the fight winds down, he even begins to mourn his lost padawan ("You were the chosen one!") and he never speaks of Anakin in the present tense again. It is unclear if he has started to tell himself the murder story yet, but in his mind, Anakin Skywalker is dead.