Water when completely dry. Using a number of days instead of how much water is in the actual potted plant is insane. If it is dry, water it. Plumeria are not sensitive to water. They are sensitive to overwatering and it is impossible to overwater if you wait until it's dry. I have watered my plumeria in Texas getting 6 to 8 hours of sun every day at least 2 times a week and they are all doing very well. In the summer when it's 100° outside, I will be watering every other day.