The phrase 'ถึงเมื่อไร (thǔeng mʉ̂a rai)' is used in Thai to ask or express the duration of an action or event, similar to the English 'Until when' or 'How long'. This phrase is typically placed at the end of sentences. The word 'ถึง (thǔeng)' means 'until' and 'เมื่อไร (mʉ̂a rai)' means 'when'. So together, they are used to inquire about the end point or duration of a particular action or situation.