1-I haven't seen you these days. Are you alright?
2-I haven't seen you for ages! Nice to see you again!
3-I went to a Sushi restaurant after a long interval.
It is a good question, but difficult for me to make difference among them.
Yes, it's difficult to know the difference! But it's good to change from "Long time no see" which is the answer most people give. Here are some other good expressions...some you can use for more than one situation...
1. "I haven't seen you around recently, how are things?"
"Where have you been hiding?"
"I haven't seen you around the office, everything OK?"
"What's been happening?"
"What's been going on?"
2. "I haven't seen you for ages, what's been going on in your life?"
"OMG! How long has it been?"
"How have you been?" (Since the last time I saw you...)
"It's been ages, are you well?"
"I haven't seen you in years, how are you keeping?"
3. "I haven't eaten here for ages."
"It's been yonks since I last ate here."
"The last time I ate here was years ago."
"I haven't been here for such a long time."
Good answers by the way Lab san! But..."haven't seen you recently" is best. Or "I don't see you these days". Also "...a long interval" might sound too formal. "...after a long break" would be better!