At the Apple-Samsung trial, SVP of Marketing, Phil Schiller said Apple has an internal measuring stick to judge sales of a new iPhone: "Each new generation sold approximately equal to all previous generations combined."
That quote comes from The Verge's live blog. CNBC reporter Jon Fortt tweeted Schiller said it was an internal joke. So, this probably isn't some rigorous statistic Apple uses to determine success or failure of the iPhone.
However, that doesn't mean it's not an interesting measuring stick to use for the iPhone.
For the first four generations of the iPhone, Apple easily sold the previous generations combined. For the iPhone 4S, and the forthcoming iPhone 5, it seems like it will be a much bigger challenge.
As you can see in the chart in this post, Apple has only sold about half the iPhone 4Ss it needs to sell to hit the cumulative sales target. If Apple continues to sell the iPhone 4S over the next few years, it should be able to reach that milestone.
As for the iPhone 5, Dediu thinks Apple will have to sell over 263 million to hit the same target. Dediu is skeptical Apple can hit that number. Maybe if it sells the iPhone 5 for as long as it sells the iPhone 3GS, Apple could sell over 200 million, but it's not going to be easy.