My wife and I did that exact circuit last Thursday on the way to Vegas and then back again this Sunday (I'll post a more detailed account when I get a chance). On Thursday we were the only car at Harris Ranch, Tejon ranch, and Barstow (we got to Harris Ranch at 11:25 am, we got to Tejon at 1:47 pm and we got to Barstow at 5:17 pm).
On Sunday at 12:26 pm, we were the second car at the Barstow charger (located behind the Chilis) and two more cars pulled up shortly after we hooked up to the charger (every other charge bay at Barstow shares a stack of chargers so I parked next to the charging car and got a very nice 263 mi/hr charge rate). We walked across the bridge to the outlet mall and my wife did some shopping while the car was charging. Several tour buses had just dropped off a horde of foreign shoppers and the place was completely packed. By the time we got back to the car, all four cars were still charging and another was waiting. The white Model S that was waiting grabbed our spot the instant we left. Tejon ranch has six charging bays and there we only saw one other car the whole time we charged on Sunday (which wasn't long at all since we got a very healthy charge at Barstow and Harris Ranch isn't all that far away). At Harris Ranch, once again we only saw one other Model S charging while we were there. There are a total of seven charge bays (six new ones near the Harris Ranch Restaurant and the old one next to the gas station).
Anyway, the short answer is that I doubt there will be much of a wait at either Harris Ranch or Tejon. But you might have a short wait at Barstow if you hit it at a popular time.
Harris ranch is pretty compact so everything is only a short walk from the chargers. But many of the restaurants at Tejon ranch and the shopping outlets at Barstow are a bit of a walk from the chargers so if you're traveling with other people and don't want to just sit in the car while its charging, you might want to drop the others off at the restaurant or mall prior to plugging in the car.
