I've been on doxycycline for 6 months for hair loss. How long should I continue?

Patient with LPP require immunpsuppressing and immunomodulating mediations for various length of time. For some it's short for others it's many years. A physician who follows you closely can guide you. The decision on what types of medications to use and how long comes from an up close examination as well as review of your symptoms and overall hair loss.

This is a question you want to ask the Dr. that has you on it. Make sure you speak directly with that Dr. and come up with a reasonable game plan.

You should be treated under a supervisor of a doctor. If you are being treated by a doctor, you need to listen and follow up with your  doctor for your questions and concerns. 

Since you are under the care of a doctor, I would go back and see your doctor and ask this exact question.