Did you check the defrost heater itself.

Looks like your heater is the glass type and they do have problems wirh burning out.

How long it takes for the evaporator coils to ice up so bad that it effects the temperatures is dependent on a lot of variables.

1. How often the doors are opened
2. What is the humidity like where you live
3. How well do the door gasket seal
4. How much food is in the freezer

And probably some I have not thought of so it is possible that it could run OK for quite a while.

Also you have replaced the other two components in the defrost system.
The defrost thermostat and the control board so all that is left is a bad connection or the heater.
Note I am assuming that the main control board controls the defrost. I am not 100% sure of this as getting tech info off the net on GE's is next to impossible.
Could also be you received a defective part.
You may want to check that the door switch is working correctly. On most units it is the opening/closing of the door that determines how often a defrost cycle occurs.