How long can a 9v battery charge an iPhone?

Making a charger and I'm trying to fun it how many charges can I get off a 9v battery. Anyone know?

Anywhere from zilch to loads.

It all depends upon the charge capacity of the 9v battery and the efficiency of the conversion used to drop the voltage to the 5v required to do the job. It also depends upon which iPhone model and how much life the battery has left; different models have different charge capacities and over time the rechargeable batteries will gradually hold less charge than they did at their peak performance.

In short, you're asking a real "How long is a piece of string?" question.