I doubt there is any definitive literature out there on this, so answers will be somewhat subjective. I think the maturity of the child is the primary factor in this kind of decision. Barring laws against it and extreme weather of any kind:
Within reach (as in, you prop open the door and sit within a few feet while they finish a nap or pretend to drive the car) - any age
Within sight (as in, you are in the house but can see them through the window, the car windows are down and the car is inaccessible to strangers) - age 7 or 8. This is the age we allow children to be in the library where I work without having their parents in the same area. It is an age at which children can call out for help.
Out of sight but reachable by cell phone - 12 to 14, which is the age at which you might leave them alone at home. 12 to 14 year-olds are capable of using the cell phone and of locking the door to safeguard themselves. Duration depends on maturity and the location of the car. Some children would be okay left in pairs though not alone.
In all cases, the keys are not in the car with the child.