The capacity of a lithium battery, how much power it can hold, is measured in milliamp hours (abbreviated as mAh).
All devices that contain a lithium battery have a mAh capacity rating. This can be found on the device itself or, in the case of a smartphone, in the settings, or by searching your phone make/model online. To find out what model of smartphone/tablet you have, click here.
Although discharge rates may vary, generally speaking, a 1,000 mAh capacity lithium battery will take about an hour to fully discharge.
In addition, the number of milliamp hours in a device will also tell you how many times the device will charge your smartphone or tablet.
For example, if you have a smartphone that contains a 3,000 mAh battery, and are using a 10,000 mAh mophie powerstation to charge it, you will get about 3 full charges and a partial 4th.
(10,000 / 3,000 = 3.33)
This does not account for a slight transfer loss, but the math will be consistent across all power products and smartphones.
To learn more about the characteristics of a Lithium Battery, click here.