Question

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?"

Answer 1

Better yet: How long have you known Peter? OR How long have you known him?

Sra

Answer 2

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.