For comp, I would give yourself something like 3 months prior to go off. It takes your body at least a month to readjust itself to natural hormone levels again. The last thing you want just before a comp is yet another variable that you can't predict. Probably the biggest issue with short-term effect of B/C is water retention. Long-term, it will probably play some small factor in your ability to lean out. Some people it affects, some it doesn't.
Do you notice any significant issues with reducing bodyfat now? I went on b/c about 15 yrs ago and stayed on until 4 months before my first natural comp. I never really noticed huge fat retention problems before but I also had never dieted down that far either. But if your goal is to lean out as far as possible, I would definitely lose the b/c once you go into your pre-contest training.
I would also note that in researching this question on other boards, I've seen girls claim to compete while on b/c with no problem. As with all hormone-related issues, its a very individual thing. You might consider going off for 2-3 months now to see what happens (while holding your diet constant enough to at least see if the hormone variable is a significant contributor to your body's estrogen response).