As the PP said, it varies by restaurant, by time of day, by CM, etc. I've mostly heard that it's somewhere under 15 minutes and then you either go into the walk-up queue and/or are considered a no show for your ADR.
I do remember being at BOG and hearing the CM tell a group they were 16 minutes late to the ADR and could not longer be seated. They were welcome to check back later in the evening to see if anything had opened up, but that BOG was currently fully booked. The group left, so I don't know what happened to the "no show" fee, but it is something to keep in mind.