Mayonnaise should not be left to sit out at temperatures above 50 degrees Fahrenheit for more than 8 hours. After 8 hours, eating mayo can pose a threat of food borne illness.
An opened jar of mayonnaise can be safely stored in the refrigerator for 2 to 3 months after the printed best-by date. After the best-by date, the mayo loses some of its flavor and quality, but may still be safe to consume.
Mayonnaise can be stored unopened in the pantry for 2 to 3 months after the best-by date. To maximize shelf life, mayonnaise should be stored in cool and dry locations or refrigerated.