An avocado turns brown relatively quickly after being cut, when an enzyme called polyphenol oxidase comes into contact with oxygen in the air. There are several ways to minimize the effect and keep your cut avocado or guacamole dip fresh for longer. How you handle it will determine the time before it turns brown.
The Trio Approach
When cut and left at room temperature, an avocado typically turns brown after about six hours. Ways to extend the freshness and slow down the browning process include keeping the pit intact; covering with a nonporous top; and rubbing the surface of the avocado with lemon or lime juice. Use all three methods simultaneously to keep a cut avocado fresh, with minimal discoloration, for up to two days in the refrigerator.
H2O
If you are working with avocado in a dish that will be served within a few hours, water is your ally. Avocado pieces immersed in water will stay fresh for up to four hours. For a shorter time, simply rinsing them with water helps delay browning. Save any leftover guacamole by pouring about half an inch of lukewarm water over the top of the dip, then covering and refrigerating for up to three days.