I happen to see a sentence using "since when" and I try to summarize its usage as follows:
1. since when + auxi V + S ?
usage:used for showing that you are surprised or annoyed by what someone has said or done, because it does not seem right, fair, or sincere.
Ex. Since when have you ever been concerned about my feelings?
(Từ lúc nào anh lại quan tâm đến cảm nghĩ của tôi ?)
Lưu ý: Trong trường hợp này động từ dùng được nhiều thì khác nhau.
2. ……………, since when + S + V …….
Ex. The film was produced in 1979, since when the cultural management system has witnessed great changes.
= The film was produced in 1979. Since then, the cultural management system has witnessed great changes.'
Source: from native English Speaking people
Do you agree ? Please share with me.Thanks