The longest I think they'll last is another two or three months.
I don't think they'll last for much more than a few months.
この場合、次のような言い方ができますよ。
ーThe longest I think they'll last is another two or three months.
「彼らは長く持ってもあと2〜3ヶ月だと思うよ」
ーI don't think they'll last for much more than a few months.
「彼らは数ヶ月以上は持たないと思うよ」
ーDo you want to bet on it?
「賭ける？」
ご参考まで！
I don't think their relationship will last more than a few months. Want to make a bet?
こんにちは！
ご質問ありがとうございます。
ご質問について、いくつか言い方が考えられますが、例えば、
I don't think their relationship will last more than a few months. Want to make a bet?
とすると、『彼らの仲は、[持って](https://eikaiwa.dmm.com/uknow/questions/39545/)数ヶ月だと思うよ。賭けようか？』となります。
参考になれば幸いです。
カナダ 
日本 
