posted by Franco .
Can you say: "How long is it since you have known Peter?" Isn't since always followed by the past simple? Thank you for your cooperation.
I corrected the alternative: "How long have you known him for?"
Better yet: How long have you known Peter? OR How long have you known him?
If you want the verb in simple past tense, then use the verb "meet."
How long has it been since you met Peter?
The simple past is used for a single action in the past; it's over and done with.
The present perfect (with helping verbs "have" or "has") is used for an action that started in the past and continues into the present.