When the 格助詞（かくじょし）
に is used, sometimes it has the nuance of ‘frequency’. This means that it can be utilized to describe how often something happens/is done. In this grammatical construction, the ‘time frame’ is followed by
に, then followed by the number of times that you would like to say that something happens.
In this way,
に keeps its regular nuance of being a ‘goal’, or ‘destination’, within which something else takes place.
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私（わたし）は一ヶ月（いっかげつ）に一回（いっかい）友達（ともだち）と遊園地（ゆうえんち）に行（い）く。
I go to the amusement park with my friend every month. (One time per month)
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彼（かれ）は一週間（いっしゅうかん）に2回（かい）しか歯（は）を磨（みが）かないから、口（くち）が臭（くさ）い。
Because he only brushes his teeth twice every week, his mouth stinks.
This use of
に should not be confused with
で in relation to time. When
で is used for ‘time’, it is expressing the amount of time that is required to do something, or how much time something is expected to take. It is not used for frequency.
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2年間（ねんかん）で50万円（まんえん）も貯（た）めた。
In two years, I saved up ￥500,000. (Natural Japanese, as this expresses the amount of time that something took)
Due to this, you can think of
に as marking a portion (or portions) of time inside of a larger space of time, within which something happens. However,
で marks a whole space of time, within which something can be/is completed.