The above expresses frequency(i.e., how many times) of an action performed in a given period of time. 回, the counter for the number of times, can be replaced by 度.
一しゅうかんに 二回か 三回 かぞくに てがみを 書きます。
I write to my family two or three times a week.
四年に 一度 オリンピックが あります。
The Olympics are held every four years.