How much of this gum do you have to chew until it starts to work?
How much of this gum do you have to chew before it takes effect?
ーHow much of this gum do you have to chew until it starts to work?
「効き目が出るまでにどのくらいのガムを噛まなきゃいけないの？」
to chew で「噛む」
to start to work で「効き目が出はじめる」
ーHow much of this gum do you have to chew before it takes effect?
「効果を発するまでにどのくらいのガムを噛まなきゃいけないの？」
to take effect で「効果を発する・効果を生じる」
ご参考まで！
カナダ 
