The deadline depends on the country where the dividend was paid and the statute of limitations in that jurisdiction.
In Switzerland, you can reclaim up to 4 years' worth of DWT refunds, as the statute of limitations includes the current calendar year plus the three prior years.
Sprintax has obtained concessional treatment from the Swiss Tax Authorities for ESOP participants that allows you to include the current year in your review.
For dividends paid in 2023, the deadline to reclaim the withholding tax would be December 31, 2026.