I have had a tupperware of sauerkraut I bought from the grocery store in my refrigerator for almost two months. When/Will it ever go bad?
Yes, saurkraut can go bad. It is a fermented product, protected from other micro-fauna growing by the acidity of the juice, a natural pickle created during the fermentation.
Having the juice cover the solids is important to protect them from spoilage.
If the juice gets diluted, or there simply is not enough to cover the kraut do to spillage, being eaten or whatever, then molds and other things can colonize and begin to grow.
Throw your saurkraut away if it shows signs of mold, or if it has an unusual odor.
Still, because of the pickle, it should have a long shelf life in practice. Still Tasty suggests six months, refrigerated.