My hatches so far this year, all fresh eggs and set on the same day, have hatched over a four-day period, during which time I've opened the incubator once to remove the dividers on day 18 and on day 22 and 23 to remove chicks before shutting the incubator down on day 23. I always open unhatched eggs and, from a batch of 80, find a few infertile and a handful of late(ish) death in shell, but since the overall hatch rate is generally good and there was nothing I could've done about the others .....