I haven't seen you for a while. I miss you.
How are you doing? I haven't seen you for an age!
Our acquaintance is of long standing―of many years' standing.
the state of not having heard from someone in a while
It is a long time since I saw you last.
It is [has been] a long time since I saw you last.
長い間.
How are you doing? It's been a long time since we last met, hasn't it?
It's been so long! Finally we have met.
I was kept waiting for an eternity.
I've been waiting for that for a long time.
wait (for) a long time