Since when do you know each other?
vs
Since when have you known each other?
I think people use the first option more often.
however since is a signal word for present perfect.
Learn English – Since when do you know each other? VS Since when have you known each other
present-perfect
Best Answer
connotes doubt or even incredulity that the two people in question know each other at all. "Since when" is synonymous with "How long has this been true without my knowing it?" The asker of the question is less interested in finding out the length of time than in communicating their surprise.
asks when the two people in questions first met. It would normally (but not necessarily) be answered with a specific moment, e.g. "Since 2013" or "Since yesterday"
Another alternative is:
This would normally be answered with a duration of time, e.g. "For three years", or "For a couple of days".