for后面跟时间段，单数、复数都可以，例如：

He was in hospital for a week and laid up for a month after that.他住院一个星期后又卧床一个月（摘自《柯林斯英汉双解大词典》）（本句中，for后面跟了时间段单数）

He struggled against cancer for two years.他喝癌症抗争了两年（摘自《牛津词典》）（本句中，for后面跟了时间段复数）