To have a window to do something means a) to have a period of unbooked time in your diary or schedule. 'I have window on Friday at 10am . Do you want to meet then?' or b) a period of time in which an activity can or must take place. 'They have a window of an hour to free the hostages. Then it will become to difficult.'

window of opportunity...maybe How about a whole sentence?

it means to have time in your schedule or an opportunity to do something