... AFIntel;
Great question ... and I see that you are a new guy in the Forum(s) ...
... So, let me first say ... Hi and welcome to the FSM Community Modeling Forum(s) ... I feel certain that you will find that we are a very friendly and diverse group of scale modelers. I am glad that you stopped in and joined up ... I hope you are too.
Now, as to your paint question ... I am not sure exactly how long bottled model paint can remain "good". A lot of my paints don't get to hang around long enough to go bad ... I somehow use them up ... you know ... the "standard" colors that we all use a lot of. As for the other colors ... the ones I don't seem to use often ... I think some of them have been on the workbench for more than 10 years. But, I think it has a lot to do with how the bottles are handled ... what conditions they are stored under ... and perhaps even the color.
I think your practice of adding BB's and mixing them about once a month will go along way to extend the useful life of your bottled paints.
Perhaps some of the other Forum members have some other ideas/experiences.