Is there a shiur how much one must eat in order to satisfy the condition of borer that it must be eaten right away? i.e. can one take a half teaspoon of the food and then leave the food to be eaten later, or must one eat a normal portion of food right away?
The idea of taking food from the waste (“pesoles”) is that the act must be an act of “eating” rather than an act of “separating.” Provided that the person is eating the food that he wants to eat, he is considered “eating” and not “separating” and therefore not in violation of Shabbos.
Therefore, it is only permitted to separate the food from the remainder if the food is being eaten. It is not permitted to separate a large amount if one is only eating a small amount, unless one can only eat the small amount by means of separating a larger amount.
It is permitted to separate a large amount for the coming meal where there is a chance that this large amount will be eaten. Once the separation has been made there is no obligation to eat the whole amount.