Is it supposed to rain today?

It is supposed to rain in the afternoon.
午後から雨になるみたい。

Is it supposed to rain today?
きょうは雨が降るかしら。

How long is it supposed to last?
時間はどのくらいかかるのでしょう？

Good medicine is supposed to be awful. If it tastes awful, that means it works well.
いいお薬はまずいものなの。まずかったら、それはよく効くってことよ。 

"A man has to know when to quit." "Is this what you are supposed to say after dragging it so long?"
「男は引き際が肝心だ」「それが何年もぐずぐずしたあげくに言う言葉かね？」

This isn't supposed to be fun. It's supposed to be work.
これは遊びじゃないぞ。仕事なんだ。