This is a very common question and people are generally wide-eyed with surprise when they hear the answer.
Floating in water does NOT mean an egg is "bad"
Eggs float when the air cell in the egg becomes larger. An older egg generally has a larger air cell from the albumin (white) dehydrating but it can also be caused by storage in a dry environment or by poor egg shell quality. You can see how large the air cell of an egg is by "candling" or shining a light on the egg. The size of the air cell is one factor for determining the "grade" of the egg but it has no affect on whether the egg is edible or not.
How do you know when an egg has gone "bad"?
There is no mistaking the obnoxious sulfur smell of a rotten egg. If it smells nasty, it is a goner - no question. The bad odor is caused by bacterial invasion. Pink whites when raw are another sign of bacterial contamination and should be discarded as well. Cloudy whites indicate extremely fresh eggs as the carbon dioxide has not separated from the white yet. If there is a brown spot or speck on the yolk, that is a normal occurrence and not caused by bacteria. Beware - rotten eggs can explode so handle with care!
How long does it take for an egg to go "bad"?
It depends on how it was stored. Unrefrigerated eggs will deteriorate faster than refrigerated eggs because refrigeration slows the growth of bacteria. Mother Earth News did a study on various methods to store eggs and found refrigerated farm fresh and store bought eggs were still "good" when their study ended at 7 months. Yep, SEVEN MONTHS!
Our informal test ...
I sell the fresh eggs to our customers in the winter and we eat eggs gathered in the peak of "Egg Season" (March to May) of the year before. This year, we packed some away to see just how long they would last. After 10 months in the refrigerator, our eggs looked and tasted better than premium brand store bought eggs. By 1 year, the eggs were dehydrated, almost all yolk & the yolk was sticking to the shell but our barn cats thought they were delicious.
Bottom line is that eggs are "good" until they are not & refrigerated eggs last longer than unrefrigerated eggs.
Buy eggs "in season" and store them in a refrigerator. Eggs are plentiful from March to May and are the highest quality of the year at that time. This is the main reason we decided to offer Home Delivery as the farmer's markets are not open during the real "egg season".
If you are not sure ... Check your eggs by cracking them individually in a separate bowl rather than throw them out to reduce food waste and save money in your food budget. They look more like a goopy or smelly mess than an egg. Estimates are that 30-40% of our food supply is wasted and much is due to people discarding "good" food based on "expiration" dates rather than food quality.
Eggs are "in-season" now - tiz the season to stock up!