once a year 前面要加for吗

前面可以不用，例如：

1、The Tourist Board inspects all recommended hotels at least once a year.旅游局至少每年视察一次所有举荐的旅馆。

2、The snake sheds its skin once a year.蛇一年蜕一次皮。

3、Many people have made it a rule to detoxify once a year.许多人已经养成了每年进行一次身体排毒的习惯。