As Christopher says, when you use 'since when' with the present tense, you are not talking about time. You are unhappy with the other person's behaviour, and you are expressing your disapproval or even accusing the the other person of what he/she has said or done.
"Honey,since when is our marriage a problem?!"---Our marriage has never been, and isn't now, a problem.
"Since when are you a therapist?"---Don't talk to me like that. You are not a therapist.