The arrival window is the forty to sixty minutes between a confirmed booking and the therapist reaching any central Amsterdam address.
Because the therapist travels to the client, a same-day booking is given an arrival window rather than a fixed minute. At this house that window is forty to sixty minutes from confirmation to the knock on the door, and it is the same for every central address. A hotel on the canal ring, an apartment in the south of the city and a room near the eastern docks all fall inside it. The house does not publish a separate figure for each district, because in practice the difference is swallowed by the time it takes a therapist to get ready and set out.
The window starts when the booking is confirmed, not when the first message is sent. The office normally replies on WhatsApp within about ten minutes, agrees the details and, where a deposit applies, sends payment details. Once the deposit has cleared, the therapist leaves. So a new client messaging at eleven at night should expect her at roughly midnight rather than a quarter to twelve.
A booking made in advance for a named time does not use the window in the same way. The time she arrives is the time written into the confirmation.
Schiphol and the airport hotels are served by arrangement rather than inside the standard window, since the journey and the building access vary more. Those bookings carry a deposit, the journey time is agreed with you before anything is fixed, and they are wisely made with some notice; the Schiphol page explains how they are arranged. Addresses beyond Amsterdam and its airport hotels are outside the service area.
Keep the phone used for the booking within reach, since your therapist may message when she is close, particularly if the building has a night entrance or a lobby desk. A correct room number in the thread saves several minutes. The tonight guide covers late bookings in more depth, and the WhatsApp booking entry describes the confirmation step that starts the window.