what time do you typically have to arrive for first come first serve spots before they fill up?

If you can pull off arriving Friday night, I'd recommend it. Otherwise, aim for 8a or 9a.

Depends...weekdays (mon - thurs) are no problem. Weekends, however, especially in the summer and fall, get quite full. There day weekend and holidays can fill up completely. Personally, I shoot to get there by 3p on a typical Saturday and expect to have to settle for a small selection of sites.