Wiktionary
adv. 1 (lb en colloquial rhetorical question sarcastic) From what time. 2 (lb en colloquial rhetorical question sarcastic) (as an interrogative interjection) Used to indicate doubt as to the veracity of a statement.
Wikipedia
Since When is a 1998 album by Canadian rock band 54•40. It marks the band's return to the more acoustic folk rock sound of their 1980s albums. The album was the highest charting album in the band's history, peaking at #19 on the RPM Canadian Albums Chart. Also, the album's lead single and title track, " Since When", is the highest charting single in the band's history.
"Since When" is a song by Canadian rock band 54-40. The song is the first single and title track of the band's eighth studio album, Since When. The song is the highest charting single in the band's history, peaking at No. 11 on the RPM singles chart in Canada. It has since become one of the band's most popular songs.
Usage examples of "since when".
It's not doing a bloody bit of good as far as cooling me off, but since when have London summers been known for their refreshing qualities?
It is, in fact, one of the few businesses here run by women, since when one is too old or loses one's looks she becomes a manager of younger ones or a housekeeper for them or something like that.