1) Both "When did you last see each other?" and "When did you see each other last?" are correct. 2)" How long has it been since you haven't seen each other?" "How long has it been since you last saw each other?" Both correct. 3) "How long is it since you last saw each other?" Technically, this is not correct. Each other ----> I defer to Viviana's comments. ----- So I am guessing the situation that you are describing is that he saw his family this morning (and now it's the afternoon). But before this morning, he had not seen his family for a while. In this case, we use the pluperfect (past perfect tense) : "How long had it been since you last saw each other (when you finally met again this morning)?"

Hey Simone! That is correct you can say these to one person or multiple people and you can say them even when he met his family this morning: When did you last see each other? How long is it since you last saw each other? (this one is not preferable since you should use the past perfect instead, but people still say this) It would be better to say: How long has it been since you last saw each other? BUT for the other sentence it should be: How long has it been since you HAVE seen each other? The question that sounds the best is: How long has it been since you last saw each other? You can also say: When was the last time you saw each other? When did you see each other last? Hope that helps!

We use the reciprocal pronouns each other and one another when two or more people do the same thing. Traditionally, each other refers to two people and one another refers to more than two people, but this distinction is disappearing in modern English.