If you were to date someone, how much difference in age would you be comfortable with?
この場合、次のように言えます。
ーIf you were to date someone, how much difference in age would you be comfortable with?
「誰かと付き合うなら、何歳までなら大丈夫？」
how much difference in age ... を使って歳の差を聞くことができます。
ご参考まで！
What would be the maximum age difference you could accept for your partner in a romantic relationship?
ご質問ありがとうございます。
"What would be the maximum age difference"＝「何歳差が最大になりますか」
☆（又は、"What would be the biggest age difference"とも言えます）
"you could accept"＝「あなたが受け入れられる」
"for your partner in a romantic relationship?"＝「恋愛関係の相手として？」
ご参考に。
カナダ 
アメリカ合衆国 
