i didn't understand in preposition it has writern use for when the noun refers to time duration

The preposition FOR is used to indicate time or duration here means that we use FOR when we measure time(over a period of time). For example:

• She's lived there for seven years.( FOR, here indicates that she has been living there from past seven years till now )
• He held his breath for seven minutes.(FOR,here indicates that she has been holding his breath from past seven minutes and is still holding it now).

Hope it helps.