If you were to write the TOEIC test, how much time do you think you would have left over after you finish?
You're a native English speaker, so I wonder how long it would take you to write the TOEIC test.
ーIf you were to write the TOEIC test, how much time do you think you would have left over after you finish?
「もしあなたがTOEICを受けるなら、どのくらい時間が余ると思う？」
how much time do you think you would have left over ...? で「どのくらいの時間が余ると思いますか？」
ーYou're a native English speaker, so I wonder how long it would take you to write the TOEIC test.
「あなたはネイティブだから、TOEICを受けるのにどのくらい時間がかかるんだろう」
I wonder how long it would take you to ... で「あなたが…するのにどれくらい時間がかかるんだろう」
ご参考まで！
How many minutes do you think you’d have left if you took the TOEIC?
・How many minutes do you think you’d have left if you took the TOEIC?
TOEIC受けたら何分くらい余ると思う？
・If you took the TOEIC, how much time would you have left?
TOEIC受けたらどれくらい時間余る？
→ minutes にこだわらず「時間」で聞く自然な言い方です。
・How early do you think you’d finish the TOEIC?
どれくらい早く解き終わると思う？
カナダ 
アメリカ合衆国 
