Under normal circumstances it's probably accurate to within a month.
They are trying to group similar cars together, but not so many they run out of a certain part. That's where the order of the numbers can get really screwed up. If they serialize a car that gets option X and then before it gets scheduled to be built, they run out of the parts for that option, the car gets put on material hold. It could stay there for months. So if it was serial # 000123, it could be built 2 months after serial #'s 000122 & even 3 months after # 000124.