Charging times differ between charging models. USB charging is limited by the maximum current output of the USB device, and may take longer than charging with conventional chargers.
However, if your phone clearly seems to be taking too long to charge, please bring the device and the charger to your local Nokia Care Point for inspection.
Note: all batteries including high-quality batteries like Nokia's will eventually lose power retention capacity. If it is time to replace your battery with a new one, be sure to purchase an original Nokia battery from an authorized Nokia dealer.