Re: how much training before...
I agree with David, that you should have one foundation art and maintian that foundation. A year and a half ago I took a few BJJ classes, after about 9 years of aikido. I found that I really wanted to just focus on aikido, that even with my flexible schedule, I didn't have the time and energy to practice two arts seriously. Also, I didn't like the schedule of the BJJ classes (they ended at 9pm, which made it hard for me to wind down in time to get enough sleep).
There are more people on aikiweb who support cross-training than I've seen in the aikido dojo where I've trained. Most aikido shihan that I've encountered seem to encourage some training in iaido (or sometimes kendo), to augment your aikido, but among the masses here BJJ is often seen as a complementary art. I am a traditionalist, I guess, so next time that I cross-train I hope it will be in iaido or kendo. Either way, I think that after shodan is a natural time to begin cross-training, even though I know that standards for shodan vary widely.