I'm thinking about taking a lesson with A. I haven't had a lesson with him for a long time.
I'm debating whether or not to book a lesson with A. It's been a while since I've taken a lesson with him.
ーI'm thinking about taking a lesson with A. I haven't had a lesson with him for a long time.
「A先生のレッスンを受けようかな。しばらく彼のレッスンを受けていない。」
この場合、haven't ... for a long time「しばらく…していない」を使って言うのが自然かなと思います。
ーI'm debating whether or not to book a lesson with A. It's been a while since I've taken a lesson with him.
「A先生のレッスンを予約しようかな。彼のレッスンを受けてからしばらく経つ。」
It's been a while since ... で「…して以来しばらく経つ」＝「久しぶり」という言い方もできます。
ご参考まで！
カナダ 
