None!, you might just get 3/4s of a charge from a flat phone battery (if you turn it off to charge), but if you start to charge with it still on you might just get over half to maybe 3/4s of a charge.
I bought this
QIDIAN 1A/1.5A/2A/4A, 6 - 18650 Mobile Power Bank
because of this
Review of USB battery box 6x18650 Qidian
I’ve charged my S4 at least 4 times before having to recharge, and i’ve used it for other things inbetween also.