difference between when "ตอนไหน" and when "เมื่อไหร่"
What are the differences between when "ตอนไหน" and when "เมื่อไหร่". I know "ตอนไหน" is colloquial but are there any other differences for example in how you answer a question with either of these words?

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I think ตอนไหน is more about what time of the day you are going to do something. For example, จะโทรมาตอนไหน when will you call ? answer : ตอนเย็น  In the late afternoon. เมื่อไหร่ is used interchangeably, but there is a sense of a larger time span, it can be years , weeks, etc. For example, จะมากรุงเทพเมื่อไหร่  when will you come to Bangkok ? answer : ปีหน้า next year

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ตอนไหน is a spoken language but you can use it in informal writing as well. When it comes to formal writing, you use เมื่อไหร่, instead. เมื่อไหร่ can be used in speaking too.