how long does it take the average person to be a "good" boxer??

the same amount of time it takes a person to be a good mechanic, or a good mathematician. Good is a highly subjective term. How proficient a person is at any given subject will be directly related to how much effort is given toward mastery of that subject.

That being said, I don't think a person can outright claim that you can be a good boxer after (X) amount of time.

An example: I have been training in the martial arts for the last 13 years. Do I consider myself good? Sure, but what is my basis for comparison? What is my control? Somebody who never trains? Somebody who dabbles?

Further than that, in my 13 years of training, have I mastered it? Of course not...I learn daily how little I actually know. Now, the persons that I consider masters of my art I would think of as being 'good.' Am I of their caliber? Again, of course not, but I still think I'm good...however, in comparison to them, I am inept. So, you have to consider the huge variability of the measurement 'good.'

Depends on how you consider good. Then it depends on how athletic and quickly you learn. After a year of training I was able to beat guys who were bigger and stronger then me, however, they were never trained to fight. Beating your average joe and being good are two diffrent things, well depending on your definition of good. Just stick with it.