It depends on the situation.
Example 1: - no "since when"
Let's say you work in a factory.
I come to you and ask you:
"How long have you been working here?"
The question is fully okay and fully neutral.
2 days later I see you in another factory.
This confuses me, and I ask you:
"Since when do you work here??"
Example 3: - no "since when"
You ask your teacher:
"How long have you been a teacher?"
This would be correct.
You discuss with a friend about cars.
Your little sister (imagine it) begins to speak with you about cars.
You and your friend laugh and ask here:
"Since when do you have any knowledge about cars?"
Do you see?
There is a clear difference between Example 1+3 and 2+4
This "since when" is more a question when something is unexpected.
Do never use "since when" when it's a neutral question.
P.S. Not a teacher