The arrival window: forty to sixty minutes, counted from yes
Forty to sixty minutes is the span between a confirmed hour and a knock at a central Amsterdam door, counted from the moment the hour is agreed.
Every question about timing on this site resolves into one figure. Once an hour is agreed in the thread, a therapist reaches a central Amsterdam address in forty to sixty minutes. It holds at nine in the evening and at two in the morning, and it holds in every district the two of you are likely to be staying in.
Where the count begins
Not with your opening message. Two people usually exchange a handful of lines before an hour is settled: what the two of you want, which lengths are free, who is free. The window opens at the end of that exchange, not the beginning, and a couple planning against a boarding time should add the conversation to the journey rather than assume the journey covers both.
Why it is a span and not a number
Because she is crossing a working city. Bridges open, trams stop, a street by the water is closed for a morning. A single promised minute would be a fiction, and a range that has held for years is worth more than a number that would be wrong twice a week.
The airport is measured separately
A room at a Schiphol airport hotel is not inside the sixty minutes. Those bookings are arranged one at a time, with the journey time agreed with you before anything is fixed, which is the arrangement the page on the fly-in day describes in full.
Reading it backwards
On an ordinary trip the window is a detail. On the night before a sailing it is the whole plan, because it is what turns a boarding hour into a last sensible start time. Subtract the transfer, the check-out and a real night of sleep from the morning, then subtract the window from what is left, and the hour to write in the first message is sitting there. The embarkation guide does that sum on paper.