It's been a long time since/How long has it been since......

It's been a long time since/How long has it been since......
 

I always get tongue-tied in Spanish when I first see someone I haven’t seen for a while because I have never been sure how to say:

“It’s been a long time since I’ve seen you.”

Are any of these correct? or What do you suggest I learn to say?

Hace mucho tiempo que no te veo.
Hace mucho tiempo que no te veía.
Hacía much tiempo que no te veía.

“How long has it been since we’ve seen each other?”

Are any of these correct? or What do you suggest I learn to say.

¿Cuánto tiempo hace que no nos vemos?
¿Cuánto tiempo hace que no nos vimos?
¿Cuánto tiempo hacía que no nos veíamos?
¿Cuánto tiempo ha pasado desde que nos vimos?

Are there any grammar rules I need to be aware of to convey the above?

Any comments, grammar rules, or suggestions to help me clear this up is appreciated.

They are mostly right, but conveying different messages:

[by phone] -¿Por qué no nos encontramos el próximo sábado? Hace mucho tiempo que no te veo.
The other two are very similar. The one with "hacía..." celebrates the fact the "no verse" situation has ended.

The other group is correct, with some of them being more factual (when imperfect is avoided) and others having an emotional nuance.

One I've been saying is (correct me if I'm wrong, aleC) Ha llovido mucho desde que nos vemos

It's correct with the meaning "It's been raining a lot since we started dating".

[by phone] -¿Por qué no nos encontramos el próximo sábado? Hace mucho tiempo que no te veo.

The other two are very similar. The one with "hacía..." celebrates the fact the "no verse" situation has ended.

The other group is correct, with some of them being more factual (when imperfect is avoided) and others having an emotional nuance.

Thank you, aleCcowaN. The above comments help to understand.

Another way to say it is this:

Wow, it's been a long time! = ¡Cuánto tiempo sin vernos!

It's not really a question, it's just expressing that it has been a long time since you've seen each other. If you actually want to ask how long it has been since you've seen each other:

¿Cuánto tiempo hace que no nos vemos? = How long has it been since we've seen each other?

If you're looking for the Mexican variant, you can substitute "tiene" for "hace" ¿cuánto tiempo tiene que no nos vemos?

If you want a more informal, idiomatic way of saying it:

¡Qué milagro que te dejas ver!

which would be something along the lines of "long time no see" or maybe "you're a sight for sore eyes!"

Related:

It's wonderful to see you! = ¡Qué gusto verte!

It goes more along the lines of "you've really made yourself scarce!" said with some hint of sarcasm. Or used when someone visits some place he or she usually avoids. Or as a recrimination for a long absence, this time involving indeed the enthusiasm of "you're a sight for sore eyes!".

Yes, "Qué milagro que te dejas ver" does hint at a bit of an accusation, somewhat along the lines of "Don't be a stranger".

Another translation for "you're a sight for sore eyes" = "benditos los ojos que te ven!"

For “you’re a sight for sore eyes,” I normally say “dichosos los ojos que te ven.” I heard it in Spain many years ago, but I don’t know if it is only said that way in Spain and not in Latin America or not. Nevertheless, it’s another great way of saying the same thing.

"Dichosos los ojos (que te ven)" is commonly said around here too. :)

Yes, "dichosos..." here too.