5 Days, GCNP... Yup, that'll do it!
5 days is more than realistic for a holiday to the Grand Canyon from Southern California. Like I said before, it will take about 1/2 to 3/4 of a day to drive to the South Rim of the Grand Canyon by I-40, so if you plan to explore a bit I'd say a full day, arriving in GCNP in the early evening. You'd then have 3 days to explore the National Park at your leaisure, and even the surrounding area, leaving a full day to get back.
Personally, I'd do the exploring on the first day, and wrap the trip up with just a straight shot back home.
-Brad