Student or Learner
"How long has he had this car?"
"How long has he been with you?"
"Since when has he had this car?"
"Since when has he been with you?"
Unfortunately, "since when" has a meaning as an idiom which can confuse your third and fourth sentences. I would go with 1 and 2.
for how long have you believed Since when do you have the right to tell me what to do?But really, since when is $250 too expensive for a nice watch?Usage notes: used in the form of a question when you do not believe a particular attitude or situation is right or true
See also: since
Cambridge Dictionary of American Idioms Copyright © Cambridge University Press 2003. Reproduced with permission.
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