It takes a very long time (half a day) to charge up with a USB 2.0 charger plugged in the wall outlet.
The USB standard only specifies a rather low charging rate (0.5 amps), but there are well-accepted ways for devices to get a higher rate (by negotiating with the host). See How much power does a Galaxy Tab 10.1 charger need to supply? for more details (of course the specifics about the Galaxy Tab probably won't apply to your phone, but the general background info probably will). If your tablet came with a special USB adapter, try using that to see if it charges faster.
As Sagar notes, turning as much stuff or putting the tablet in sleep mode will help recharge rates.