What is the minimum headache or migraine days per month that you would consider starting preventative migraine therapy?

I would start thinking about preventive therapy when headaches get to be about 4-6 times a month. This is the number at which there is evidence that headaches will start to progress to chronic, which is considered to be 15 times a month.

If a patient is increasing their abortive use or each migraine...

4 migraines per month should start the discussion of preventive therapy. It may be considered in less than that if the patient has very disabling or prolonged migraines, where they are missing work or school, and it is disruptive.