I just came back the other way, left UCSD at 10:30 on Monday of Labor Day weekend, but it was pretty quiet. I also opted to take the Coastal route instead of dealing with the I-5 traffic, not fast but a lot more fun.
If you don't have the Waze app I'd suggest getting it. Google Maps (which feeds Waze) says taking I-5 at 9AM will run typically 1 h 30 min - 2 h 20 min and taking I-15 will typically 1 h 50 min - 2 h 40 min.