Your Tier lasts through the end of the following calendar year (eg. reach Platinum anytime in 2026, and you keep it through December 31, 2027).
Your Tier Points balance resets to zero each year on January 1, so you'll need to re-earn the threshold each year to hold onto, or move up, your Tier (eg. you reached Platinum in 2026, you keep it through December 31, 2027, and then you must earn the Platinum points again to keep your status for 2028).