How long have you been getting your hair cut at that hair salon?
How long have you been going to that hairdresser?
この場合、次のような言い方をすると良いですよ。
ーHow long have you been getting your hair cut at that hair salon?
「どのくらいその美容院で髪を切ってもらってるの？」
to get one's hair cut で「髪を切ってもらう」
ーHow long have you been going to that hairdresser?
「どのくらいその美容師さんとこに通ってるの？」
to go to that hairdresser で「その美容師のところに行く」
ご参考まで！
カナダ 
