2

Although the answer by @Colin Fine is correct, it doesn't completely cover the usage. Example John: Bill, your grammar isn't good, I think you should study more. Bill: Huh! Since when are you an expert on grammar? I get better marks than you do.


2

It's a common informal way of saying "I don't believe that is correct". On the surface it is saying "As far as I know, X is correct. How long has what you said been right?" It is slightly combative, but less than saying directly "That's wrong".


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