Reciting Qunut (like during Subuh) in the last rakaat of witir begins on the night of 16 Ramadan until the end.Reciting Qunut is an act that is recommended, but if you do not do so, your witir prayer is still valid.

For the Hanafi Mazhab, it is deemed as sunnah to recite Qunut on every last rakaat before ruku’ (bowing) when praying witir, be it in Ramadan or outside of Ramadan.

[1] Al-Nawawi, Al-Majmu’, j. 4, pg. 24

[2] Al-Kasani, Bada’i al-Sana’i, j. 1, pg. 273.