Depends on the battery capacity of the phone. Find out what that is, multiply it by 3. Could be anything from c.4500 - 15000 mAh.
The Anker ones I've owned seem sold items.
Its amazing what Ah ratings some sellers put on their battery packs (particularly ebay Chinese sellers)
My business background is 'electrical' and I have a fair bit of knowledge in low voltage (battery) applications.
A bit of background :
1Ah is (basically) the ability to run an item at 1 amp for 1 hour, or 2 amps for 30 minutes, or 4 amps for 15 minutes etc etc.
1mA (mille amp) is 1/1000th of an amp, so 1000mAh is 1Ah, 10,000 mAh is 10Ah etc.
A typical car 'starter battery' will be in the order of 80Ah
Knowing all of this I purchased a 30,000mAh (30Ah) 5v Solar panel & battery power pack off ebay for about £12
Remember the 80Ah car battery - and the size and imagine the weight that would be ?
How big is a 30Ah battery power pack + Solar recharging panel ?
Answer - about the size of a packet of cigarettes and total weight of about 230g.
It is physically impossible (and contravenes all known science) for that battery power pack to be 30Ah (30,000mAh)
I undertook some tests and to cut a long story short it was actually just about 4Ah
My phone battery is rated at 1400mAh so I can get just under 3x recharges on my phone.
My GPS battery is 2700mAh so I get 1 & 1/3 recharges
My Headlight is 1200mAh so I get just over 3x recharges.
I then purchased (again off ebay) a 2600mA 5v power bank, did all the testing again and it actually turned out to almost exactly 1Ah (1000mAh)
Do not be fooled by the quoted ratings - I don't know how they measure them, or how they get away with such claims.
The 30,000mAh solar charging battery bank