About one hour, barring any traffic construction , tree trimming, rumble strip insertions, menehune sightings, vistors stopped and taking pictures of chickens, and vehicle accidents.
Early is a good plan for the canyon. Tip: Look up there toward the mountain, and see if it is clear. No rain clouds.
Actually, if you have your copy of the Ultimate Kauai Guidebook, on the inside back page, lower corner is a great sliding scale for driving time in minutes for most of the very popular towns and sites.
Dont forget to visit the Kalalau Valley lookouts as well. You will have some marvelous photo opportunities....have fun !
Edited: 4:55 pm, February 12, 2013