It is a misconception that you will start to cough up tar after smoking cessation. I can imagine why you would think that - after all, cigarettes contain tar and that has to have gone into your lungs, so it has to come out, right?
What happens is that you breathe in fine particles with smoking, part of which your body gets rid of in the period after you had that cigarette. Another part settles in your lungs, but it's not going to get out. I'm guessing you're visualizing it as a collection of tar/dust in your lungs which you're going to cough out eventually if you keep coughing up enough. It simply isn't. You get mucus because of irritation/inflammation of the lungs due to smoking.
What you are going to find is that your lung function gets better with time after smoking cessation. The amount of mucus you cough up will likely also get less. This review gives some interesting background information about what smoking cessation does to your lungs and body.