Are you wanting to do one monthly water change? Or several smaller changes?
I have not heard of a magic percentage per month-as every tank is different. I would suggest going by your tank parameters. Whatever percentage it takes for you to keep your ammonia, nitrates, nitrites, etc. where they need to be. Keep in mind, this may need to be done on a weekly basis--biweekly. Water changes helps to export nitrates out of your tank. Just depends alot on your tank bioload.
Hope this helps
Jolene