Dvd write speed and elapsed times on 3500



I was wondering if someone could tell us at what speed and elapsed times a full dvd is burned at.For example at 4x how long should it take.Same question goes for the other speeds.
The reason I ask is I dont see a big decrease in time for an increase in speed.Also does the 3500 adjust the burn speed as required per its media info?I seem to remember reading that the 3500 stops during the burn to check the media.Will it therefore have a slower burn with poorer quality media even if I set the burn speed the same?