“For” or “During”

We use during to say when something happens, if it happens in or over a period of time.

We use for to talk about the length of time something lasts.

COMPARE:
They went to Florida during the winter. – They went to Florida at one point in the winter.

They went to Florida for the winter. – They went from the beginning to the end of the winter.

Example:
She phoned me during the week to tell me that she was getting married. (referring to a point in the week)

We were in the cinema for three and a half hours.
Not: … during three and a half hours.