One Month
noun
Remember the 〜ヶ月 counter? Replace the 〜 with a one and this becomes one month.
一 uses the on'yomi you learned with it, but gets shortened to いっ, so watch out for that. The 月 reading is the げつ on'yomi you've learned. You can guess it's げつ here since it's not being used in a specific month's name.
クリスマスまであと一ヶ月ですね！
It's one more month until Christmas!
一ヶ月半、日本で学校に行くんです。
I'll be going to school in Japan for one and a half months.
一ヶ月間、家から出ずにゲームをした。
I played games at home without going outside for a month.