Karenさんの文章二つも合っています。
"I haven't seen your face for a long time."
"I haven't seen your face in a while."
"for a long time" と "in a while" の代わりに "in some time" や "for some time" も言えます。
"I haven't seen your face for some time."
これも加えます、
"It's been some time since I've seen your face."
"it's been some time" 「しばらく」
"since I've seen your face”「あなたの顔を見たから」
[しばらく](https://eikaiwa.dmm.com/uknow/questions/46861/)についてもっと詳しく知りたい方はクリックしてください。
"あなたの顔しばらく見てないな"
- I haven't seen you in a while.
- It's been a while since I last saw you.
- Long time, no see.
I haven’t seen your face for a long time.
I haven’t seen your face in a while.
合ってます。冗談みたいな感じ？
通常は "face" ではなく "you" だけを使うけど、これでも大丈夫です。
オーストラリア 
アメリカ合衆国 
