Do you recommend taking vitamin B6 to prolong a luteal phase of 9 days? What dosage? At what point in the cycle should it be started?

Vitamin B6 has not been proven to increase the length of the menstrual cycle or luteal phase. It is likely just a coincidence that your cycle was longer when taking B6. It is best to focus on maintaining a healthy diet and taking folic acid prior to conception to support fertility. If you have concerns about fertility, it is recommended to seek advice from a healthcare professional for appropriate guidance and potential treatments.