When to buy a new Smartphone?
Simple answer: If you feel you need it and you have the money, then NOW. There is no better time than now. (And that applies to everything in life, we live only in today, not in tomorrow, nor yesterday. ONLY in TODAY.)
In average the prices drop 30% each 6 months, so the best time to buy is 1-2 months, 5-6 months, 11-12 months after release, when there is a notable price decrease. Major releases happen between September and November. Also in general a good time to buy is right after Christmas, when many sales are going on.