Mine went from 89% to 95% in 8-10 seconds on 100% charged iPhone. The iPhone was still 100% charged when I unplugged the Pencil.
I will try to completely deplete the pencil today/tomorrow to see how long it takes to fully charge from the iPhone and how much of the iPhone's battery it depletes.
Kinda cool using the iPhone as a "Mophie" of sorts, since Apple doesn't advertise it (at least, not that I've seen).