When someone that you know well does something new that they've never told you about before, it can be surprising. When you're surprised by a friend, coworker, or acquaintance's actions in this way, you can say "Since when do you ___?"
For example, if a family member who usually doesn't listen to you suddenly asks for your opinion, you can say:
Since when do you care what I think?
If your friend who's never studied Spanish before tells you that he bought an Spanish grammar book yesterday, you can say:
Since when do you speak Spanish?
It's important that you understand this: you don't ask "Since when do you..." if someone tells you about something new that they're doing. You say this if they mention something that they've already been doing for a while, but which you've never heard about before.
Also, notice that each of these examples uses "do you" instead of "did you", even though they kind of describe the past.