How Stable
 

Is Sta-bil, as in what is it's shelf life?

I don't know, because I've lived in some of the most temperate zones on the planet (North Da-Fukcin'-Dota) and never had the need to use it.

I've had bottles for over a year that I used in the diesel fuel for the boat with no problems.

The stuff that is supposed to help with ethanol in marine engine fuel.

Still don't help with it phasing out of the gasoline.

...how would I know...lately people at work have noticed a slight instability occurring with me...

I think it's a solution looking for a problem.
I've let stuff set for 2-3 years with a half tank of gas in it and had it fire right up and run fine.
Hell, my tiller gets run so little, the gas in it is 4 years old. No problems.