How long to BB?
I think James Hawkins is probably about the fastest to black belt on this forum. That's in years, not necessarily time on the mat. He's been challenged on it several times but there are plenty of guys who've trained with him that will attest to the fact that he's the real deal. I don't think he would qualify as average, though.
On the other end of the spectrum, I started in Kenpo in 1973 and finally earned my BB last year. For a huge chunk of that time, I was in the Air Force, moving around with no opportunity to advance. Many of those years, I simply remained proficient. I have a lot of years IN kenpo but not so many AT kenpo. So, I'm guessing it would be somewhere between 2 and 34 years, depending on the level of effort and dedication.
How long to be able to defend yourself? Against whom would be a good question. I felt that I was able to defend myself pretty well by the time I earned my purple - 6 months or so. After that, it's a matter of who you are defending yourself against, what kind of training or weapon they may have.
So many variables.
Take it out on the heavy bag,