It depends on the reservation. Most ADRs just require the credit card to guarantee against no shows and nothing is charged unless you do not show (some folks have reported a small charge being placed and then later cancelled, mostly likely the system's way to determining if the card is valid). However there are some reservations that require full payment in advance (e.g., CRT and HDDR come to mind). For those, the full amount of the meal will be charged to the card as soon as the ADR is made.