From my 20 years of experience with Testors. I still can't forecast what one bottle will be compared to the next. I have had flat black that wasn't flat. Some bottles last just a few months, whereas I have some bottles that are 20 yrs old. I think the storage environment is the biggest issue. I think a lot of my paints may have been frozen or exposed to extreme heat when being shipped since I used to order through mail order. Also, paints that I use a lot are exposed to evaperation more hence they may gum up and dryout quicker.
I do see a difference in quality between Testors and Floquill, Gunz Sangyo, and Tamiya. I have experienced that Floquill has had the longest shelf life of all my paints no matter what the color was. I am inpressed with the tins that WEM paints come in. I have paint that is in tins that my mother gave me. Some of these enamels are over 30 years old and still in great condition. So I think the container and how airtight the lid gets has a lot to do with the life of paint.
Scott