An open container of store bought mayonnaise will last between one and three months in the refrigerator. Sources vary widely on the shelf life of unopened, commercially-bottled mayonnaise stored in the pantry -- from one week to four months past the sell-by date. Keep in mind that the date printed on the container refers to how long the mayonnaise will remain at optimum quality, not the date by which it will spoil.

Homemade mayonnaise has a considerably shorter shelf life compared to store bought mayonnaise, as little as two days and up to two weeks. Most recipes for homemade mayonnaise call for raw egg; in addition, homemade mayonnaise lacks the preservatives that contribute to commercially-bottled mayonnaise's longevity. You can substitute pasteurized eggs to minimize the risk of foodborne illness and extend the life of your homemade mayonnaise.

Regardless of the date printed on the container or when you prepared your homemade batch, if your mayonnaise appears to have spoiled, throw it out. Signs that mayonnaise has gone bad include a change in color or texture, usually from creamy and white to lumpy and yellow. If mold appears, discard the entire container. A foul odor is another sure indicator that your mayonnaise is spoiled. Never taste your mayonnaise if you suspect it has gone bad as doing so could cause serious illness.

Whether you're making a sandwich or a chicken salad, don't abandon your efforts if your mayonnaise has spoiled. There are several great alternatives to mayonnaise, such as plain yogurt or plain greek yogurt; for even more flavor, add a few drops of lemon juice and a pinch of salt. Depending on the application, other options may be cottage cheese, whipped cream cheese, or sour cream. For a tuna or chicken salad, consider a totally different approach, such as basil pesto in place of the mayonnaise.