**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
- For Teachers