Once you reach a level, it’s yours for 12 months, no matter how much you spend during that time. If you move up to a higher level, your 12 months start again from that day.
At the end of your 12 months, we look back at what you’ve spent over that period. If you’ve reached the spend for your level, you keep it for another 12 months. If you haven’t, your level will move down to match what you’ve spent.