How long would 2200mah portable battery last on iPhone 4? - 1

Hey I'm looking to buy a iPhone 4 portable battery charger as I'm out for long hours and my phone always runs out of battery I'm looking at the 2200mah but I don't know how much battery life that would give me?

Divide the phone's battery capacity into the portable battery's capacity to get a rough estimate of recharges the phone can get. Now this would be from a new fresh battery, not an older worn out battery.