I'm not sure how long.
I think I figured out what the problem was.
I was taking the temperature in one of the corner air holes(temp 92). I moved it to the opposite side and the temperature was 98.
I turned it up and one side was 101 and the other side was 95.
I figured either one side of the heat element failed or there was another problem.
Yesterday I had laid the 20 eggs due to hatch on one side and all the eggs that were in 10 days or less on the other side. The collective embryonic metabolism of the older embryos raised the temperature of one side affecting the thermostat and lowering the temperature on the other side.
As a remedy, I pulled all the eggs, put the mature ones in a ring around the outside of the incubator and put all the others in the middle. Temperatures seem to be a lot more consistent throughout.