"since when" could be a sharp reply to a question.
"So, you think that you are the boss? Since when, who died and made you God?"
When used in a more conventional sense, it is just part of a question.
"How long (this is the sense of desde cuando, which I understand to mean - from what date or from what time) have you worked here? I've been here since 1987."
Thank you, Gillnetter.
The problem I see with "how long... ?" is that the common answer would be a period, for example: "I've been here for 24 years". "How long?" is more like "¿Cuánto... ? or "¿Hace cuánto... ?"
So, how do I ask if I want the answer to be the date? Maybe in English there is not an exact form to obtain as the answer the date something began.
"From what time (date) have you worked here?" Is that correct?