Yogurt usually has a relatively long shelf life, but at what point does it become unsafe? It really depends on the expiration date stamped on the container, which is either a "sell by" or "use by" date. Regardless, never eat yogurt that has a blue-green tint or smells or tastes unusually sour.
“Sell By” Date
Most yogurt is stamped with a “sell by” date rather than an expiration date. The yogurt should be good for about two weeks after the “sell by” date, according to Julie Garden-Robinson, a food and nutrition specialist at North Dakota State University.
“Use By” Date
Some brands stamp a "use by" date on their containers instead. Do not eat yogurt beyond this date because it could be unsafe.
After a container of yogurt has been opened, it should be tightly covered and eaten within 10 days. Keep in mind the expiration dates mentioned above, because they still apply.