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I think it varies. Let me give some examples. Let's say I'm talking to my wife and she says, "Have you filled out that warranty form for the new TV?" and I reply, "Yes, I have." Then she might say, "When did you do it?"

"When were you doing it?" wouldn't make sense here. Now let's look at an example when it would.

I see an article online about going vegan. I say to my wife, "That would be fun to try." and she says, "Oh I did that once before I met you." and then I say, "Oh! I didn't know. When were you doing it?" and she says, "When I was doing ballet in high school and wanted to feel healthier."

To be super clear, "when did you do it?" is like asking "when did you complete this specific thing?" while "when were you doing it?" it kinda like asking "when were you doing that thing" aka what time period.