The key factors that affect the discharging time of a power bank are battery capacity, power conversion efficiency and discharging current.
The unit of capacity is mAh, which means the discharging time based on a settled discharging rate. Current discharging rate is conducted on capacity, such as a 5000mAh lithium-ion battery, if discharged at 1C(5Ah), the discharging time is 1 hour; if discharging at 0.5C(2.5Ah), then discharging time is 2 hours. For different digital products, discharging current from same power bank may be different, but the dicharging voltage keeps same around 5V. Battery capacity is the key factor affecting discharging performance, the discharging current decides how long the discharging time maybe.
Conversion rate refers to capacity consumption rate during voltage boosting from 3.7V to 5V, during which original battery capacity would be consumed by parts on the PCM, thus the final capacity discharged would be smaller than the capacity stated. Generally it's a perfect boosting system if conversion rate could reach 85% or above, it means around 85% the originally capacity we can discharged.
Maintanence and proper use would also affect the usage of a power bank.