〜に〜回 details

Examples

per — a count or an amount measured against one unit of time

英会話えいかいわのレッスン、週しゅうに一回いっかいだけ受うけてるんだ。

eikaiwa no ressun, shuu ni ikkai dake uketeru n da.

I only take an English conversation lesson once a week. casual

この薬くすりは一日いちにちに三回さんかい、食後しょくごに飲のんでください。

kono kusuri wa ichinichi ni sankai, shokugo ni nonde kudasai.

Take this medicine three times a day, after meals. neutral

一週間いっしゅうかんに十時間じゅうじかんくらい日本語にほんごを勉強べんきょうしています。

isshuukan ni juujikan kurai nihongo o benkyou shite imasu.

I study Japanese about ten hours a week. neutral

当館とうかんでは月つきに一度いちど、館内かんないの点検てんけんを行おこなっております。

toukan de wa tsuki ni ichido, kannai no tenken o okonatte orimasu.

Here at our facility we carry out an inspection of the building once a month. formal

Formation

期間きかん＋に＋回数かいすう／分量ぶんりょう（一日いちにちに三回さんかい・月つきに一度いちど・一週間いっしゅうかんに十時間じゅうじかん）attaches to noun phrase (schematic)

Notes

に drops when the period is a bare 週・月・日 and the count fuses onto it —
週三回しゅうさんかい, 一日三回いちにちさんかい — which is the default on
labels and common in speech; a counter-phrase period keeps it (一週間に三回).

Related patterns

Similar

につき
につき belongs to notices and contracts and distributes over a unit of entitlement (お一人ひとりにつき二枚にまいまで); 〜に〜回 is the everyday spoken shape and measures against a stretch of TIME, so a period noun is what precedes it.
ごとに
ごとに counts off each successive unit and so fixes the spacing (三日みっかごとに = every third day, evenly); 〜に〜回 gives only a rate inside the period and says nothing about when within it the occurrences fall.

Lookalikes

に
The same に, but a period noun in FRONT of it turns the phrase into a denominator — 二時にじに飲のむ pins a clock time, 一日いちにちに二回にかい飲のむ gives a rate; it is this frame only when what precedes に is a stretch of time and a count follows directly.
おきに
一日いちにちに一回いっかい and 一日いちにちおきに look parallel and mean different things — the first is once every day, the second is once every OTHER day, so おきに skips a unit where に counts inside one.