A/C problems while idling

I have a 2006 Toyota Prius that the A/C does not blow cold air while the car is idling or at a stop. However when I'm driving above 30 mph it works like a champ. Has anyone had a similar problem? Any suggestions on how to fix this? My compressor does have a humming noise, is that normal? I already checked the condenser fans and they are working/pointing the right direction. By the way, does anyone have a clue on how many ounces of A/C oil (ND-11) the system requires? Thanks in advance and hopefully someone can help me out of this nightmare.