I'd say (as a non-local, two-time summertime visitor) that one full day at SDC is perfectly doable but should be your bare minimum. I've done one full day with a late closing, and one evening plus day with Trailblazer on the full day, and both were relaxing trips which included the cave tour and lots of rides and meandering around. I don't think it's quite at the level where a one-day trip would feel overwhelming or disappointing, but it's certainly worthy of more if desired!