All these changes happened over a period of time.

所有这些变化都是在一段时间内发生的。

This crisis might last for a long period of time.

此次危机可能会持续很长一段时间。

She spent a period of time working with people dying of cancer.

她用一段时间帮助即将死于癌症的人们。

Oil or vinegar based salad can be stored for a long period of time without refrigeration.

含油或醋的沙拉可以储存很长一段时间，而且不需要冷藏。

The money was lent for an undefined period of time.

这笔钱无限期借出。