When charging a Power Bank it depends on a few factors as to how long it will take to charge.  Firstly the charger used makes a difference in charging time.  Different chargers have different power output strengths.  All chargers with a USB port are 5V output however the output amperes or milliamperes (mA) will vary.   For example:

• Computer port 0.5A or 500mA output
• Most wall chargers 1A output or 1000mA
• iPad wall chargers have a 2.1A or 2100mAh output  and are not suggested for charging cheaper power banks.

A second factor is that the Power Bank itself has input power regulating technology built in.  This is a feature to protect the battery.   Most power banks have a maximum of 1A input power rating.

The third factor is inefficiency in the charge process.  20% power is lost during the charge and discharge process.

PowerBank charge time Example:
Powerbank: 2600 mAh  (0% remaining)
Power Supply/ Input: 1000mA plug
Calculation: 2600mAh/ *800mA = minimum 3.25 hours
(*Why 800mA? An estimate of 20% power is consumed during the charging/ discharging process)

Same calculation using PC port, 0.5mA:
Powerbank: 2600mAh (0% remaining)
PC USB Port : 500mA
Calculation: 2600mAh/ 400mA = minimum 6.5 hours
(Why 400mA? An estimate of 20% power is consumed during the charging/ discharging process)