1. Haven't seen you for weeks.
2. Long time no see.
3. It's been (such) a long time.
4. How long has it been?
5. It's been too long.
6. It's been ages since we last met.
7. I'm so happy to see you again.
8. Fancy meeting you here.
9. You haven't been around much lately, have you?
10. It's always a pleasure to see you.
11. Where have you been hiding?
12. How come I never see you?
13. What have you been up to all these years?
14. How've you been all these years?