Depends on the gate keeper. San Bernardino county campgrounds have a ridiculous 3pm check in time even check out is the standard 11am. I arrived at Yucaipa around 11am and it was no issue. I was able to go to my empty site. I arrived about the same time at Prado Dam and was greeted with a “Well, you’re early.” and was directed to a parking lot and was told I could occupy my site, which was already empty, at 3pm. You’ll really like Yucaipa.