Unless you plan on speeding along I-5 which easily can be done it will take longer than 8 hours. Doing close to the speed limit with no stops is 9.5 hours and that is with no traffic in Las Vegas and entering the Bay Area.

Throw in a few bathroom breaks and lunch and you have even more time added to the trip. I've driven from San Francisco to Las Vegas and back every year for the last 8 years and my best time is 6 hours and 15 minutes. I MIGHT have pushed the BMW E39 5-Series with a 6 speed to the top speed of 155 mph for a distance on that trip. Most trips driving with traffic took about 9 hours with a break for lunch and bathroom breaks.