Tricky question, I was going to say 2 hours like PP, but thought I'd double check using google maps. It confirmed the about 2 hours, but also included estimates with traffic up to 3 1/2 hours, which I can fully understand. That time of day you may hit rush hour traffic, which will increase your time dramatically.
Although it's not the shortest route, I would take I-10 to I-15 to CA-60 since that will avoid some potentially nasty traffic through Riverside. This adds about 10 miles to the trip, but could save an hour of driving. I would plan on leaving about 6:00, that should get you to DL sometime between somewhere about 8:00. If you leave later you will run into more traffic.