When defining the amount of food that needs to be eaten – either the amount needed to fulfill a commandment or the amount needed to determine whether a person is liable for punishment for having eaten something that is forbidden – we must look not only at the volume of food, but also at the amount of time during which the act of eating is performed. Generally speaking, in order for an act of eating to be considered significant, a person must consume ke-zayit be-kheday akhilat peras – an amount the size of an olive in the amount of time that it takes to eat half a loaf of bread.
The source of the term peras is from the root that means “to break,” so the actual meaning of the word is “a broken piece,” or, in this case, half of a loaf.
All agree that the peras in this rule is half a loaf of bread. What remains unclear, however, is how to determine the size of the loaf. Rashi believes that the loaf is the size of eight eggs, so the peras would be four egg-bulks. The Rambam rules that the loaf is the size of six eggs, so the peras would be three egg-bulks. Beyond that disagreement, we also find arguments about the volume of an egg – which may be as large as 100 cubic centimeters or as small as 40 cubic centimeters. Furthermore, do we estimate the time that it takes to eat this amount based on the average person or is there a subjective standard for each individual person?
From the continued discussion in the Gemara, it appears that the length of time under discussion is equivalent to the amount of time that it takes to walk half a Roman mile (mil), which is estimated at about nine minutes, while in other places it appears to be the amount of time that it takes to walk 200 amot, which is estimated at about two minutes. Practically speaking, the different opinions as to the halakha range between those two extremes.