How to predict the life of Chrome Plating?

You are absolutely correct. Salt spray is a QC check and not a predictor because of the infinite amount of environments and uses.
That said, 1 month is a sorry plate job for 99% of chrome uses.

Alas, this is the nature of corrosion testing. It's only good for process controls (e.g., testing to see if your process is behaving today as it did 6 weeks ago). It does not give you any idea as to actual performance in the wild.

You can develop a correlation, but it would take time. You'd have to vary your process, develop different runs, put some parts in some salt spray testing, and put others in actual use. Then you could determine a correlation. However, even that would be for a very specific set of conditions and would not necessarily apply to other situations. (Think South Dakota vs. Florida.)

There are accelerated corrosion tests, but none of them will actually tell you how much will a specific item last under specific conditions. They are just ball park ideas.

Dear Richard,
The deposit on the handle must be Nickel-Chrome plated and is generally mentioned as Chrome plating.The life of the deposit primarily depends upon the base metal preparation and thickness of the deposit mainly of Nickel plating say more than 20microns.In your case it may be 5 microns or less.Please also remember that failure also occurs if the atmosphere in which the product is kept is not free from corrosive fumes, Moisture etc.It is also found that the plated components rusts faster in areas very near to sea.
To overcome these platers use latest technologies in Nickel and chrome plating such as duplex/ triplex/ microcracked Chrome plating etc.
regards

There is only one true way to tell how the parts will perform and that is to directly expose them to the elements. No matter how clever we get in the lab, we never really seem to capture the random nature of the weather. Where I work we have set up a weathering station whereby we monitor the weather conditions the parts are being exposed to and just leave the parts racked by the monitoring equipment.

What this does is give you real-time exposure data, what it doesn't give you is the data of how the parts will perform in the field once they have been handled by a few people.

Sorry, no simple answer.

Hard chrome depends on the thickness of the plating. Decorative chrome would depend on the thickness of the nickel and the types of nickel plating applied before the chrome plating is applied. The same thickness of a bright nickel plate will not have as much corrosion resistance as an equal thickness of duplex nickel.
You need to discuss corrosion resistance and build up tolerance with your plater. If they cannot answer your questions you need to find a plater who can.