Hopefully the worst of the traffic will be on Friday afternoon and Monday. Good point about deciding on a stopping point for Saturday; it would be wise to call ahead and make reservations, if it isn't already too late.
I did some playing with google maps, and it looks like about 240 miles from Hawthorne to SLO, taking back-roads, but the route includes detours that add miles and time. I would guesstimate that it would take 6 hours riding time.