黎老师讲得很好，我补充几个出自《英语疑难问题解答大辞典》的例子.

■当for引导的时间短语用来修饰具有静态意义的动词是常常可以省略。

They lived in China (for )seven years.

The meeting lasted （for）two hours.

They will be in Chicago (for) a year.

■当for与all开头的名词短语连用时，必须省略for.

She’s been lying in bed all day.

■当for短语修饰动态动词时或者用于否定句是，则借此for不能省略。

I’m going away for a few days.

I haven’t seen him for three weeks.

■当for用在句子开头时，一般也不能省略。请比较：

a. He lived abroad (for) six years.

For six years, he lived abroad.

b. He waited anxiously at hospital (for) some time.

For some time , he waited anxiously at hospital.