It's a 3 months average. Although I don't know what happen if you don't have any balance at all in the previous months. I guess you don't currently have a BoA account that you could check. After you hit the status, it takes a long time before it will come down. I took my money out of the ML account last summer.. and somehow my status is still Plat Honors today. And I just cancelled my Premium Reward card after the AF posted. They took out the AF after card was cancelled.