Question 050: How much to train

I was almost raped and after much research I would like to pursue self-defense classes. However, I have a sports injury which is not too bad but can act up sometimes. I am a little overweight and would like to get back into shape so I can begin taking classes. Also my goal is to eventually become a black belt. Approximately how long do you think it would take for me to achieve that goal given I would be taking classes once or twice a week?