Its been mentioned by others, charge for parking in the stall after 60 or 90 minutes.
More than 60 minutes is too much. Particularly in SoCal, where I can drive in any direction and find another Supercharger, 30-45 minutes might be enough. But, a limit of 60 minutes is easy to figure out.
I would add that it's not 60 min of parking; the car must be charging during those 60 min, or get towed / ticketed / fees assessed.
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I charged at sjc yesterday during trip from sd to LA. Was very disappointed to see TWO owners come and plug in and have a family member drive them off. That sure sounds like locals using the chargers right during a busy Sunday. I was plugged in for 45 minutes... Those two cars were left plugged in. Disappointed in my fellow Tesla owners
I've seen this at San Juan Cap, and Culver City. I'm confident that it happens at San Diego, since the Supercharger is physically in the Qualcomm complex.
I was at one Supercharger, and the "Local" drove up and plugged in. He had out-of-state plates from a state without sales tax. I asked him if he was on vacation, and he claimed that he had those plates to scam out on California sales tax.
Then he jumped in his buddy's waiting oil burner car, and off they went.