Figure 2 days each way...
Hi uscgps
I did the i-5 both ways from north of Seattle to near Disneyland this summer. I-5 is a very good road, and moves really well -- except the the usual areas you can get traffic as you go through major cities like Seattle, Portland, and LA. And of course construction: for example, they are working on the main bridge over Lake Shasta north of Redding, which is down to 1 lane in either direction, and can be stopped if they're moving equipment or material across the road. Most of the time you'll be traveling at 60+ mph.
You can do the trip in 2 longish days each way -- the half way point is somewhere between Redding and Yreka in California, and there are a number of hotels or campgrounds in this area, with the largest number of hotels in Redding.
There are a number of places along the way you can stop, even if you just have a few minutes to an hour to get out of the car and look at something. These include a number of historical and interesting sites through Oregon (Oregon trail, Applegate trail, Rogue River, Oregon's Capital in Salem, etc.), and through California (Mt Shasta Caverns, Turtle Bay in Redding, Old Sacramento, Sacramento Rail Musuem, Olives in Corning, Sutter's Fort in Sacramento, California's Capital in Sacramento, Kern County Oilfield Musuem in Taft, Tule Elk Refuge, Fort Tejon coming into LA, etc.) including wildlife refuges, naturual sites, and etc.
If I had to pick one spot to be careful about on the trip, it's the traffic through LA. If you're arriving in the evening, you may be coming into the city right at rush hour. I-5 goes straight through the heart of LA and doesn't have carpool lanes, and if you're getting into LA right at rush hour taking the I-5 through the heart of the city is going to be very frustrating. Depending upon when you arrive, and specifically where you are heading you might consider avoiding the heart of the metropolitan area and skirting around it on the 405 Freeway (which has carpool lanes most of the way, but is very commuter clogged at rush hour), or the 210 to the 605 or 210 to 57 north- eastern loops (both with carpool lanes most of the way, and not quite as busy at rush hour).
If you're interested, my roadtrip report is at link, and while its not 100% what you're doing, I did drive I-5 both ways.