There are nearly as many rides and major entertainment/night spectaculars packed into the two DLR theme parks as the four WDW theme parks have combined.
That said, I think first-time visitors to Disneyland Resort get caught up in how many "days" to spend in each park, as if it's like WDW where park-hopping is an hour-long ordeal involving buses and boats and monorails. It's not.
DCA and Disneyland sit facing each other across a small plaza. You can walk from Space Mountain in Tomorrowland to Midway Mania in Paradise Pier in about 15 minutes, assuming the entry lines at DCA are short. You will find that you can hop back and forth from park to park with an easy short stroll, just as you would walk from World Showcase to Future World or from Frontierland to Fantasyland.
Certainly the first day you'll want to spend at Disneyland solely, and then the second day you'll likely focus on DCA. But by day three you will have the lay of the land, and hopping from park to park for parades or shows or Fastpass return times will be part of the routine of a Disneyland vacation. Not to mention that Downtown Disney is an equally short stroll, or quick Monorail ride from the Tomorrowland station, and that can open up all sorts of additional dining and entertainment options during your theme park day.
Taking all that into consideration, I would think 5 full days and nights would be a good timeframe to dedicate to the Disneyland Resort. Hopefully the person on their first-time visit to SoCal would also be able to slot in some days to get off Disney property and see some of the sights and wonders of Southern California, all within a 20 to 60 minute drive of Disneyland.
Another thing for the Disney fan to consider is the official
Walk In Walt's Footsteps Tour. This is a several hours-long tour of Disneyland specifically, and you get to see the park that Walt built from a unique historical perspective. Any Disney fan worth their mouse ears will want to take the tour, and it gives you a wonderful insight into this theme park that a man named Walt Disney built with his own hands in the 1950's, instead of the theme park in Florida that a faceless corporate committe built with smaller committees in the 1970's.
http://disneyland.disney.go.com/tours/