How long should a jar of mayonnaise be kept once it’s been opened?

Kraft recommends that opened, refrigerated mayonnaise be discarded after  3 – 4 months (or 30 days after the expiration date if that comes sooner). The USDA says 2 months after opening. (The  USDA’s recommendation refers to all brands; different brands contain somewhat different ingredients, for example, different levels of acidity.) Take your pick since this advice is based upon loss of quality not safety. After a few months, the color and/or flavor may deteriorate, but the mayonnaise will generally be safe to consume if your refrigerator temperature is at or below 40°F. 
 
It’s a good idea to write  on the label the date you open the jar so that you’ll know when the quality is beginning to deteriorate. 
 
Source(s):
Kraft Foods recorded customer service phone message 
Realsimple.com “Surprising Expiration Dates”
USDA Food Safety and Inspection Service