Please imagine after a couple of years you meet someone who didn't know much about a particular skill or in a field such as automobile mechanics or poetry or even cooking etc. His / her record showed you that he shouldn't be able to do any specific task in one of those fields, but in an occasion you find them very specialist in one of these fields. It marvels you and you are going to show your surprise and wonder. Does the self-made sentence below sound natural in this sense:

  • Since when have you become such an expert?

If not, please let me know how can you bring it up in a more idiomatic and natural way.

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    No! DON'T use your "self-made sentence" to ask a genuine query like that. It's a very common dismissive idiomatic rhetorical question that strongly implies the speaker knows perfectly well that you are not an expert. Say something like You've obviously learned a lot since [some time in the past when you clearly weren't an expert]. – FumbleFingers Reinstate Monica Jan 24 '17 at 14:23
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    More idiomatic is "When did you become such an expert?" but you have to be careful about context and how you say it, because it could imply either "When did you acquire such skill?" or, sarcastically, "Last time I heard, you weren't such an expert." – stangdon Jan 24 '17 at 15:02
  • @FumbleFingers do you mean my offered, self-made (because this is what we say in our language too) sentence is sort of sarcasm? Meanwhile what I was going to say could be sometimes sarcastic and even in order to belittle someone. – A-friend Jan 24 '17 at 19:10
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    @A-friend: Yes - particularly when preceded by Since, it's a very common facetious putdown (usually equivalent to You don't know anything about this situation, so stop pretending you do!). – FumbleFingers Reinstate Monica Jan 24 '17 at 19:16
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    As your example is "self-made", could you add how you came up with using "since when"? This is proofreading unless you tell us why you decided to write this way. – user3169 Jan 24 '17 at 21:04