I have used ProMix (cannot remember the specific type) for the past two Springs as my starter mix.
I still have a significant quantity left as I purchased a large bale.
Will this stuff still work this Spring and/or beyond? I have it stored in a huge plastic tub with lid.
I would think that it is fine. I usually buy my bales in the off-season, so by the time I get around to using it, it is at least a year old, but still sealed in the bag. I wouldn't think twice about using 2 year old Pro-Mix that has been sealed.