Egg laid to emerging time varies due to strain of bee and the temperature of the brood nest. Eggs kept in an incubator at steady brood nest temperature varied from 19.1 to 24.1 days for worker brood in a Canadian study. I have seen eggs laid on foundation that had cells with walls about 1/8 to 3/16 inches high. The workers built the cells to keep just ahead of the brood development.