Adverb
(colloquial, rhetorical question, sarcastic) From what time.
Since when do I need your permission?
(colloquial, rhetorical question, sarcastic) (as an interrogative interjection) Used to indicate doubt as to the veracity of a statement.
The moon is made of green cheese. Since when?
Since when was genius found respectable? Elizabeth Barrett Browning
Since when do you have to tell the enemy when he has won. Orson Scott Card
Writers don't own their words. Since when do words belong to anybody? 'Your very own words,' indeed! And who are you? Brion Gysin
Death is nothing to us, since when we are, death has not come, and when death has come, we are not. Epicurus
Since when did what we paid for colored cloth gauge our gravity? Brandon Boyd
Since when did a mathematician need any tools but his own head? Pythagoras had done well enough with a stick and a stretch of sand. Robert A. Heinlein