The temps will generally stay in the higher post-o category throughout pregnancy. Other than a few random highs and lows, I don't think it will start to drop at any certain time. Since you're already pregnant, there's no real need to keep charting at this point ;-)
From my experience, it stays higher than baseline if you're pregnant. This is what I've read also. Mine dipped about 6 days past ovulation, then went back up and stayed up. I stopped taking my temp, though, once I got the positive.