How long does ultrasonic rangefinder last
First off, sensing the distance once per second may be too slow even for a parking application.  Consider that 2 mph is equivalent to over 2.9 feet per second.  Almost 3 feet is a large distance in a garage. In any event, I don't think you have to worry about the reliability of the sensor. Quartz watches run continuously for years without any problems.  If the watch is analog, that means the parts associated with moving the second hand are operating once per second 24 hours per day or 31 million times per year as you have pointed out. Garage door openers have object sensors to prevent accidents that are also on continuously and last for years.