Ideally, the eggs are transferred directly to the incubator, but it’s possible to store them in egg cartons if needed. Kept at temperatures between 50 and 60 degrees and 75 percent humidity, the development of the eggs can be delayed for up to ten days without sacrificing the viability of the embryos. However, they must be stored with the fat side of the egg pointed up to keep the embryo alive. Step 3 — Incubate. It takes 21 days on average for an egg to hatch once incubation begins. Before placing the eggs inside, turn on the heat source and measure the temperature and humidity over a 24-hour period, making adjustments as necessary to create the optimal environment. If the humidity is too high or low, use a sponge with more or less surface area to adjust it. Raise and lower the temperature of the heat source in tiny increments until the thermometer reads 99.5.