Don't you think it will take some time for the cancer symptoms to appear?
I don't think the cancer symptoms will show up right away. Don't you agree?
この場合、次のような言い方が自然で良いかと思います。
ーDon't you think it will take some time for the cancer symptoms to appear?
「がんの症状が出るまでに時間がかかると思わない?」
ーI don't think the cancer symptoms will show up right away. Don't you agree?
「がんの症状はすぐに現れないと思うけど、そう思わない?」
ご参考まで!