The clock at each stop
First message to reply: 10 to 15 minutes, whatever the hour. Reply to confirmation: as long as it takes to send three details and a transfer, often only a few minutes. Confirmation to her knock: within the hour across most of W2, Bayswater and Edgware Road, and sooner still when she lives by the station. Dinner dates and overnights run more smoothly with a few hours of notice, and a Heathrow hotel needs three hours booked with travel counted in.