I think Abraham believes in chapter 12 but his faith also grows (and suffers setbacks) throughout his life, likely becoming stronger by chapter 15 and probably reaching its fullest expression in chapters 22-24 as he prepares to sacrifice Isaac, buries Sarah, and finds a wife for his son. Abraham's life is a great example of the truth that we aren't justified by the strength of our faith but rather by its object. He was counted righteous even while his faith remained, in some ways, weak.
It is not necessary to identify a particular moment of faith which 15:6 points to, and it may in fact be inappropriate to try. Certainly, chapter 15 reads as if Abraham believed at that moment. But true faith is not chiefly about any single moment. It is more important that Abraham was a lifelong man of faith; one whose faith faltered at times as is the case with all of us, but remained a constant and growing part of him.
