From what I’ve seen it takes ~12 seconds to put a permanent mark in the oleophobic coating of the phone. Less than 12 seconds but more than about ~6 seconds will leave a temporary mark that glows a bit; but will go away. Any less than 5 or 6 seconds will not leave a mark unless they are using an oxy welder to heat the area.
Sources: https://www.youtube.com/watch?v=HJYqgZDmsZo&t=232s