How much battery percentage can 2600mah charge on iphone 4s?

I have an iphone 4s and it always finishes battery. I want to know if i get a portable battery charger for my phone how much percent can it charge my phone?

Both my wife and I have separate battery packs for recharging my 4S and her 5 and they give about 3 full recharges per full 4 light indication on the pack. Mine is 7000 mah so a 2600 charger probably will only do it once but should come close to 100% on a dead phone.
You can buy cases that include auxiliary batteries. I assume you are using the phone for heavy discharge things or leaving mail checking and Bluetooth on all the time if you are constantly running out.