I switched on orders for later. How far ahead can a customer book, and how soon?

You decide both on Settings → Ordering rules under Orders for later. Days ahead sets how many days into the future the calendar shows, and Earliest slot sets how many minutes from now the first slot can be. The customer picks a day and a 30 minute slot inside your opening hours, in your own time zone. Table orders from a QR code are always for now. Booked orders sit on the Scheduled tab of the orders board until it is time to start. The Orders & delivery tab in the docs explains orders for later.