Once you reach a tier, you achieve that tier for the remainder of the year in which you earn it, plus the next full calendar year. To maintain all benefits of the tier, you must earn at least the same number of on time payments and paid off loans during before your benefits period end.
For example, if you earn Silver tier on September 1, 2018, you maintain all benefits for that tier until December 31, 2019. Between September 1, 2018 to December 31, 2019, if you earn at least the same number of on time payments (16) and paid off loans (4), you will maintain the Silver tier until December 31, 2020.