If you purchase commercially packaged, already refrigerated yogurt, you can keep it for about seven to 10 days after the “sell by” date. If you freeze the yogurt, you can get one to two months out of it. Opened yogurt will taste optimal for five to seven days after it’s opened. How to tell if it’s gone bad? Just check and use common sense. Red flags include “a highly runny watery consistency, a clumpy texture, and a sour smell.” If you see mold, throw out the whole package.