The incubation period for a guinea fowl egg to hatch is 28 days. The length of the incubation period can vary between 26 and 30 days, if the temperature or humidity is off a bit in the incubator.
The correct temperature in an incubator with a fan is 99.5 degrees Fahrenheit, decreasing to 98.5 degrees Fahrenheit during the final three days of incubation. If you have a still air incubator, set it at 101.5 degrees and decrease to 100.5 the last three days. Keep careful track of temperature and humidity throughout the incubation process, as a variance of a single degree can be problematic for the eggs.
The correct humidity for hatching Guinea Fowl eggs is 65% until day 25 and then rising it to 80% until day 28.