If you're talking about the defrost timer (supplier example only) it has a 12 hour on, 30 minutes off, automatic time cycle - non adjustable.
The principle of operation is that when the timer reaches its set "off" time, it turns off the compressor and turns on the heater to melt the accumulated ice build up from the evaporator unit i.e. the 30 minute defrost cycle time.
When the defrost thermostat reaches its set operating temp, (50°F), it disconnects the heater. (This is not the temp in the compartment, The defrost thermostat is measuring the air temp around the evap unit under which the heater has been turned on)
When the timer reaches the end of its 30 minutes "off" time it resets and starts its 12 hour "on" time again and turns the compressor back on again to drive the temp back down to the set operating temp. This may take a little while to get there, depending on how often the door has been opened and the ambient temp outside the freezer cabinet after a defrost cycle has just ended.
Once the temp reaches the defrost thermostat's set release temp (10°F), it will release so that the heater circuit is ready for the next defrost cycle. Here's a video that shows how to test a bi-metal defrost thermostat that may help.
During the 12 hour run cycle, the temperature control thermostat controls the operation of the compressor. It turns it on or off depending on whether the temp is higher than 12°F or lower the 5°F
If the temperature is not reaching the set operating temps but the evaporator coils are getting ice on proves at least that the temp is <32°F. Check if the evaporator fan inside the compartment is operating. This drags air across the icy cold evap coils (should be approx. 5°F - 12°F) and blows the now colder air throughout the compartment to cool it down to the required temperature. Be careful when checking the fan as it operates at 120V AC.
Use a thermometer to check what the actual temp in the compartment is, after the compressor has been running for a while.
If the temperature is not getting to the set operating temps of 5°F -12°F during the 12 hour "on" cycle and the evaporator fan is operating:
a) Check that the door seals are OK by placing a sheet of paper between the door and the door jamb and with the door closed normally, try pulling it out. It will come out with a bit of effort but it definitely shouldn't fall out by itself. Do this a various places around the door, top, bottom and both sides.
If the paper test fails then check the door alignment (hinges?) to make sure that the door is aligned correctly with the cabinet i.e. straight and parallel. If the seals are faulty, there are two "gaskets" (aka seals) in the freezer, the door gasket (part #5304507201) and also a gasket strip under the bottom door shelf (part #216204900).
b) If the compressor is stopping and then starting again a little while later then running for a while and stopping again etc repeatedly:
It may be a problem with the temp. control thermostat thinking that the set temp has been reached and is turning off the compressor so as to not make it colder than it should be. Once the temp rises again, due to normal user activity e.g. doors opening etc, then the compressor is restarted again to maintain the temperature.
c) If the compressor is running continually without reaching the set temperature:
Check that the condenser coils on the outside at the back of the cabinet are clean and dust free. If they're dusty etc, this reduces the efficiency of the cooling cycle making it harder to achieve the set temps as the heat absorbed by the refrigerant gas as it passed through the evaporator unit inside the compartment can't escape as quickly - radiant heat dissipation.
It may also be a problem with the sealed system. The system would need to be accessed and the refrigerant suction and high pressure values measured to determine what is wrong e.g. lack of refrigerant or a faulty compressor etc,. Depending on your location you may need a licensed repairer to do this due to the environmental regulations regarding the handling of refrigerant gases. Also they have the equipment to do this.
Here's the wiring diagram for the freezer which may also help. It also shows the sealed system pressures as well.
Apologies for the long answer but I thought that if you knew how it should operate then it is easier to work out what's happening when it isn't.