You are required to download at least 15 GB each quarter (90 days). Each quarter starts respectively on 1st day of January, April, July and October and ends ends respectively on the last day of March, June, September and December. If on the 26 day of the last month of each quarter you are bellow the 15 GB minimum, you will receive a reminder. If on the last day of the quarter you still do not meet the requirement, your account will be disabled.