This is a common mistake make by most of the people. For rough calculation of the total charge time, kindly use this formula. REAL CAPACITY * conversion rate * phone battery health / phone capacity = total charge times. 

EXAMPLE: 12000 x 0.8 x 0.8 / 1500 = 5.12 charge times (0.5-1 times plus minus) * Note that the calculation is based on normal condition whereby the power bank or device (phone/ tablet) is not in use during charging process. A running device generally consumes power therefore if your device is actively in use during the charging process, the charging performance may not meet the expectation.