The various versions of the iPhone all have slightly differently sized batteries, but are around 1400 mAh. The Waycharger has a capacity of 12,000 mAh, so theoretically it's just a matter of dividing 12,000 by 1400, but it's not that easy in the real world.
When charging a battery from another battery there is power lost in the transfer due to a number of factors. Typically it's around 25% power loss. Also the iPhone's battery typically isn't actually empty. For sake of calculation we can say it's usually around 5%. This means in the real world, the number of actual full charges is closer between 6 and 7. If you leave home with a full iPhone and full Waycharger, it's over a week for most users.