Although a combination of いつ 'when', の, 間（ま） 'interval', に, and か;
いつの間（ま）にか is a set expression that is used as a stand-alone adverb to mean 'suddenly', or 'without noticing'. Literally, it just highlights an unknown interval within which something occurred, as seen by the question marking particle, か.
3年間（ねんかん）日本人（にほんじん）の友達（ともだち）と毎日（まいにち）日本語（にほんご）で話（はな）していたら、いつの間（ま）にか日本語（にほんご）が上手（じょうず）になっていた。
After three years of speaking Japanese every day with my Japanese friend, I became fluent in Japanese without noticing.
久（ひさ）しぶりにクレジットカードの明細（めいさい）を確認（かくにん）してみたら、いつの間（ま）にか自動車（じどうしゃ）保険（ほけん）の値段（ねだん）が上（あ）がっていた。
I checked my credit card statement for the first time in a while and found that the price of my car insurance had suddenly gone up.
気（き）づいたら、いつの間（ま）にか冬（ふゆ）になっていた。
Before I knew it, it was winter.
Fun-fact - Coming mainly from the inclusion か, this grammar pattern often carries the nuance of surprise. When you want to convey the same feeling without the nuance of surprise,
あっという間（ま）に may be used instead. This is a similar expression which literally means 'in the time it takes to say あっ'.