About 6 hours? The ramps should have held. I did call them and they said not to leave a car on overnight. The guy did not give me a number, but he implied that leaving the car on for several hours would not be a problem.
EDIT: Wait a sec, you said 6 DAYS in a row? In that case, that might be it. I called their tech support, and he explicitly stated not to keep them on overnight.