It takes about 20 minutes after you start eating for the message to stop eating reach the brain. The feeling of fullness that tells you to stop eating is regulated by the stomach in 2 ways. The stretching of the stomach wall stimulates nerve receptors sending a message to the brain to stop eating. The ghrelin hormone(the "eat eat" hormone), produced by the stomach, starts to decrease.
Eating slowly allows the brain to receive the nerve and hormone messages and you do not overeat. You can test if you are full by standing up during the meal and sensing how your stomach feels. If you feel comfortable, stop eating.