The charge you will receive heavily depends on how you use your phone and its settings, including apps you may have running that could be using a lot of power as your phone is charging. It's also very important to consider the size of your phone's battery itself in relation to the juice pack.
Battery capacity is measured in mAh (milliamp hours). If your smartphone has a 2000 mAh battery, and your juice pack has a 2000 mAh battery, you can expect a bit under 100% extra battery. Why not 100%? As your phone is charging, it is still using power, so by the time the juice pack is depleted, you will have already used some of that power up.
If you have a larger phone, you might see that your juice pack case is rated for 60% additional battery life versus 100%. Why not more? While 60% versus 100% sounds like a lot less, that's just a measurement of the battery of the juice pack in relation to the size of the battery in the phone. Let's say you have a 3000 mAh phone. If you have a 2500 mAh juice pack case, you're still going to get many hours of use with your case.