Up to how long can it take for your lungs to rid themselves of tar once you quit smoking?

I quit last November, it has been 8 months and I still cough up small specks of black in my phlegm. I asked my doc. and he said that it was normal, as I had been chain-smoking 6 years prior to quitting. How long did you have withdrawal symptoms when you quit? Should I get a second opinion? I can breath just fine and all, just have excess phlegm with specks of tar in it.