Typically the drivers attention drops after 2-3 hours of driving, so it is recommended to take a break then. The legal limit for truck drivers in Europe is 4.5h of driving, after which there needs to be a break of at least 45 minutes. The maximum total driving time per day is 9 hours.
While these times are of course not enforced for private driving, they are a good measure for what is safe. Yes, you can drive 5 hours or more without a break, but you need to be aware that this is risky behavior.
Well, it's hard to maintain 80mph in the mountains, it's also less likely to be very warm as you go up. A summer trip to Tahoe from where I live in the central valley is about 2.5 hours and it easily makes that. I am even close to making it home on the same charge.
My memory of driving Washington/Iahado/Montana is that holding speed wasn’t that difficult. Lots of opportunities for acceleration and deceleration. I have never driven a Tesla. Curious to know if it can handle my driving style.
Could it make it from San Diego to Anza Borrego and back?
While these times are of course not enforced for private driving, they are a good measure for what is safe. Yes, you can drive 5 hours or more without a break, but you need to be aware that this is risky behavior.