N4
に(頻度)
に(ひんど)
Times (In each time period), Every, Frequency of
私は一ヶ月に一回友達と遊園地に行く。
I go to the amusement park with my friend once a month.
Explanation
'period of time + に + number of times' says how often something happens in that period.
「1週間に2回」 (twice a week), 「1日に3回」 (three times a day), 「3時間に1回」 (once every 3 hours).
Mixed up with
「に」 goes right after the period, and the number follows at once. Do not put 「の」 between them: not 「×1週間の2回」.
「3時に薬を飲む」 means take medicine at 3 o'clock; 「3時間に1回薬を飲む」 means once every 3 hours.
Common in medicine instructions and daily habits: 「食後に1日3回」.