when to use ‘during’ and when to use ‘for’

During is used to say ‘when’ something happens; for is used to say ‘how long it lasts.’

Example:

I was in London during the summer.
I was in London for six weeks.
It rained during the night for two or three hours.

We use both during and in to say that something happens inside a particular period of time.

Example:
I woke up during/in the night, with a severe headache. 

We use during to stress that we are talking about the whole of the period.

Example:
The school is closed during the whole of May.

We also use during to talk about an event, activity or experience [not a period of time].

Example:
I met them during my stay in China.[correct]
I met them in my stay in China. [incorrect]

He had some strange experiences during his military service. [correct]
He had some strange experiences in his military service. [incorrect]