This will depend on the state of the battery but a normal charge from a low battery to a fully-charged one will be 3.5 hours.
If the battery has become completely flat (i.e. the unit will not turn on) then you may need to do 3 lots of 3.5 hour charges, unplugging and re-plugging the USB lead in between each charge.
The reason for this is that, for safety reasons the charging circuit has a safety cut-out timer of 3.5 hours to prevent the battery from over-heating.
After the three lots of charges the unit should be completely re-charged.
We also suggest that the battery is charged using a USB Mains or Cigarette Lighter Charger when charging as the USB ports on many computers switch off when the computer is in sleep mode.