Usage of since

[Ofriendragon]

Hello everyone,
I would appreciate it very much if you could help me with these four sentences:
1. It's a long time since I saw you.
2. It's been a long time since I saw you.

3. It's a long time since I've seen you.
4. It's been a long time since I've seen you.

a. They are all correct, aren't they?
b. In sentence1 and sentence2, it would sound better if I use ... since I saw you last, right?

To express the same things as 1-4 do, can you give me, respectively, the first, "instinctive sentence " that would flash into your mind? I mean I want an alternative for them respectively if they (sentences1-4) are not often used ones.

Many many thanks in advance.

[Mister Micawber]

.
a-- They are all reasonably common. AmE in particular sometimes uses the simple present (#1 an #3).

b-- #1 and #2 do not need 'last' at all.
.
_________________
Native English teacher at Mister Micawber's

[Ofriendragon]

It helps me a great deal, Mister Micawber.