The arrival window, and what starts it
The arrival window is the forty to sixty minutes that run from the hour being agreed in writing to the knock, and it holds across central Amsterdam.
Forty to sixty minutes is the figure quoted on every page of this site, and it is worth knowing exactly what it measures. It starts when the four facts are settled in the thread, which means the address, the hour, the length and the figure. It stops when she is outside your door. Nothing between those two points is left vague, and nothing about the time of day stretches the window or shortens it.
The reason a span is quoted instead of an exact minute
A single promised time would be a guess wearing a uniform. Traffic on a bridge, a tram that does not come, a street closed for something nobody announced: any of those moves a precise minute and none of them moves a window of twenty minutes' width. Quoting the range is the honest version, and it is why bookings here tend to land inside it rather than slipping past it.
What can make it longer
Three things, all of them avoidable. An address without a floor or a room number, which turns the last two minutes of the journey into a search. A locked street entrance nobody mentioned, which can add ten minutes at the end. A phone left on silent, which can add all of them. Write the address out in full the first time and none of the three arises.
The airport is measured separately
A room at Schiphol or at one of the airport hotels is not inside this figure and never pretends to be. That journey is agreed on its own terms, with the driving time in each direction written out and accepted by you before the hour is fixed. You will always know the number before you agree to it, which is the whole point of doing it that way. How the messages run in order is laid out on the booking page.