Your membership tier is determined by the amount you spend. Once you are in the Glow Pro tier, you will be in that tier for one year from the date you earned the tier.
To reach the Glow Pro, you must spend $75 or more (excluding taxes and shipping fees) in a rolling year (i.e., a 12 consecutive month period). Once you qualify, you will instantly move up and have access to that tier's perks through the rolling year from the date you earned the tier.