How do I set the delivery time my customers are promised?

Settings → Ordering rules, the field called Delivery ETA in minutes, anywhere from 5 to 480. It is quoted in the confirmation message and on the customer's status page, so treat it as a promise in writing. It is also the number used for orders booked for later: the kitchen is told to start at the slot time minus this ETA. Change it before a rainy night rather than after. The Orders & delivery tab in the docs mentions the ETA.