my answer would be: 4.

if it really is a 100% het pied, your chance to miss out on one egg is 50%, for two eggs its 25%, for 3 eggs its 12,5%, and after 4 eggs, at 6,25%, i would write it off.

i mean if its lets say a lesser enchi possible het pied female and you want to breed lesser and enchi into pied you might still repeat the breeding to hit your lesser enchi 100% het pied. So there may be a good reason to continue.

if there isnt one, or if its really just a possible het pied female without other genes, i would stop after 4 eggs.

Also some people can make out het pied markers in possible het pieds and pick the ones that are most likely to be the real het pieds, which already casts more doubt on possible hets than would be the case with, lets say, albino.