It depends on the charging current required by the mobile device and the age of the battery in this device: * The greater the charging current required, the longer the charging time * The older the battery, the longer the charging time The USB connector of this product gives a 500mA current. Generally speaking, it would take 1-3 hours to fully charge a MP3 player. For tablet, due to a much higher charging current needed, it may take up to 20 hours.