For my money, "did" is wrong.
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JesterLegacyWhen do we use "Since when did you..." and "Since when do you...."?Do is used with actions and states:
Aspara Gus JesterLegacyWhen do we use "Since when did you..." and "Since when do you...."?Do is used with actions and states:Since when do you eat anchovies?Since when do you read The New Yorker?Since when do you believe anything he says?Did is used with one-time events:Since when did you learn to cook?Since when did you start smoking?Since when did you become such a cyni
enoonFor my money, "did" is wrong.It certainly sounds unnatural to this speaker of BrE, but there are 111 citations in COCA.