Thankful to the teachers in this forum first before I ask any questions.
for 2 questions below, are they the same and correct?
1. How long is it since we last met?
2. How long is it since we have met?
and then, how about the 2 statements below
1. it is years since we last met.
2. it is years sicne we have met.
are they the same and correct?
take your time.
**Neither a teacher nor a native speaker.**
I believe you should not use "it is" or "is it"..
You are clearly talking about past and present, that's why you should use "Present Perfect Progressive"
"It's been" or "it has been" or "has it been" - in your example sentences.
1. How long has it been since we last met?
2. How long has it been since we have met?
(Although "is it" sounds good here I'm not sure if it's also correct.)
1. It's been years since we last met.
2. It's been years since we have met.
I don't know if all four sentences are correct now.
Maybe only both #1 are correct.
Confirmation would be nice