When should we be done with our birth plan?

Work with your doc. Birth plans can be tricky. You are deciding what you want to have happen in a situation that isn't controllable. Talk to your doctor. Try to remember that this birth is one day in your life. As long as you have a healthy baby in your arms at the end of the day, that's a perfect birth experience.
As soon as possible. The most important thing to do is discuss with your doctor early about what you want and need during the delivery. That way you will have a better idea about what is feasible and safe for you and the baby. Please remember that things don't always go according to plan! sometimes emergencies arise and your doctor needs to do the best thing for you and your baby based on her experience and training.