I thought your job was supposed to finish about 6 o'clock.
I thought you were supposed to finish work around six.
I thought your job ... で始めるなら、次のように言えます。
ーI thought your job was supposed to finish about 6 o'clock.
「あなたの仕事は6時ごろ終わるのかと思っていた」
もう少し自然な言い方にすると、次のように言えます。
ーI thought you were supposed to finish work around six.
「あなたは6時ごろに仕事を終えるのかと思った」
ご参考まで！
カナダ 
