It's been 5 months since I became serious about learning English.
It's been 5 months since I started learning English seriously.
It's been 5 months since I started learning English in earnest.
SEIYAさん
ご質問どうもありがとうございます。
様々な表現があると思いますが、いくつか自然な表現を紹介します。
1. It's been 5 months since I became serious about learning English.
2. It's been 5 months since I started learning English seriously.
3. It's been 5 months since I started learning English in earnest.
4. It's been 5 months since I started learning English for real.
--- be serious about doing something = 本気で~をやる
--- do something in earnest = 真剣で~をする
--- 4番目の「 for real」はものすごく話し言葉っぽい表現で、とてもカジュアルな言い方となっています。「本気で」という直訳となります。
お好みに合わせて使い分けてみてください。
ご参考にしていただければ幸いです。
「本気で〜を始めて」は英語では"to start seriously"または"to start determinedly"のように翻訳できます。
例:
"It's been 5 months since I've started studying English seriously."
「本気で英語を勉強し始めて5ヶ月です」
参考になれば幸いです。