This is an often-asked question. The answer is that salt spray does not correlate to anything. Other corrosion tests do not correlate to real life or each other. Many have tried to study this and make a correlation, but nobody has done it yet. The problem is that the actual chemical reactions that make rust take years to occur.
Salt spray is only a predictor of real world results.
This test is also good at detecting finish defects. The best one can do is to base salt spray requirements on prior history. If you want your part to last longer in the real world, look for finishes that will last longer in salt spray. I really enjoyed the discussion of salt spray testing and what it means in the Aug. 2001 edition of Metal Finishing. That is the one question that always comes up when one teaches a corrosion class.