From Las Vegas I took a 15-hour trip/tour of the Grand Canyon - South Rim. It was fully worth it, but it was cold and windy. The temperature was ~ 40º F or less. The windchill factor might have been less than 32º F. There was snow on the ground. We got more than three hours to walk around the rim and take photos, which was more than ample amount of time. Here is my Grand Canyon photos link . I would recommend this trip to anyone that visits Las Vegas.
I visited the Grand Canyon several times, in years past. At 67 it is a very hard drive. Actually, you do not need more than three hours to see the canyon, unless you want to partake in the Rangers' activities and programs or go down to the canyon. Make no mistake abut it. The Grand Canyon is magnificent, spectacular, memorable, unforgettable, majestic, breathtaking , and many more adjectives I cannot come up with. However, after 3 hours it becomes a hole in the ground :) Well, sort of. Therefore and imho, 3 hours to see the canyon is more than enough.
To go down to the canyon and be next to the Colorado river you need more than 3 days, but t is fully worth it. You usually go down to the canyon by mule, which is an exciting, trepidating, and unforgettable experience to last a lifetime or two. I will never forget the mule going by the edge of the road and the abysm beyond the edge of the road.