The time it takes to charge a device with a power bank is dependent on a number of factors.  Firstly and most important is the output of the Power Bank, most are 1A output.  Only larger capacity power banks (5000mAh +) will have a 2A or 2.1A output.    A second important factor is the resistance of the cable used.  Some cables only have a 1A capacity capability.

The same calculation as above can be used to calculate how long the power bank will take to charge a device.  The example below assumes there is 0% power left in device battery.

Powerbank output: 1A
Phone battery: 1500mAh
Calculation: 1500mAh / 800mAh =  1.875 hours
(Why 800mA? An estimate of 20% power is consumed during the charging/ discharging process)