The only good guacamole is a green guacamole. Unfortunately, the avocado pulp in guacamole does not get along well with oxygen. The pulp will turn brown in a matter in less than a day unless you take preventive measures.
Put the guacamole in an airtight resealable container immediately after its prepared. Place a sheet of plastic wrap directly on top of the guacamole to seal out all of the air. Seal the wrap and the lid after each use.
Cover the top of the guacamole with a thick layer of sour cream, then place it in resealable container. This is an alternative to the plastic wrap method. It's the anti-browning method that should be used for single serving sizes of guacamole.
Sprinkle some lime or lemon juice on the guacamole before you put it a plastic wrap. The acidic juice helps maintain the green color.
All of these tips, even when combined, will only extend the life of guacamole to about a week.
The brown guacamole does not mean the whole dish is ruined. Scrape off the browned parts if you wish to consume the green guacamole underneath.