The Swiss FTA imposes a "statute of limitations" of three years from December 31st in the year the dividend was paid.
For example, the refund associated with the dividend paid in 2023 must be claimed no later than December 31, 2026. Your DWT refund application must be received at the Swiss FTA on this date if not before. If you mail your refund application on December 31, 2026, it will be rejected because it will be received at the Swiss FTA after December 31, 2026.