If your power bank has 10,000 mAh, you would divide that by the mAh of the device you want to charge. A quick online search can tell you the mAh of your device if you aren’t sure or can’t find the battery. For example, an iPhone XS uses a 2,658 mAh battery. Using the equation, it would take 3 hours and 7 minutes of charging your iPhone XS before your 10,000 mAh power bank ran out of battery life.