Well I charge my 4th gen by putting the wall adapter into the wall and then I carefully plug the 30 pin cable into the iPod and carefully put the USB in the adapter. Then.. *beep* it charges.
Apple website specified that it should take around 2 hours to reach 80 and 4 hours for full full charge. The full plug image might show up but in reality you will need to leave it on for longer while until it actually fully charged.
USB is required to have a minimum power output per port. Some computers put out more current, allowing devices to charge faster. Apple was probably estimating that charge time based on the minimum output.