John Gruber:

In chart form, you can see what an anomaly last year was with the iPhone 6. But given that you can almost draw a straight line connecting the other four points in the chart, I’m not willing to call it a peak yet. But even if we see a return to growth, it might take several years before we see another Q2 with over 60 million units sold.

Going based on the trend line of iPhone sales from 2012-2014, I’d guess the next Q2 with over 60 million units sold will be in two or three years.