Need a model number!!!
There are a number of ways that the time between defrost cycles is set.
Mechanical timers can advance whenever the unit is plugged in (constant) or just when the compressor runs (cumulative).
They are usually in the 6 to 8 hour range.
Manually advancing them just varies the time to the next defrost cycle or if you turn it far enough starts a defrost cycle. It does not change the time between future cycles.
Electronic also have a number of methods. One common one is by the time that the door is open but there are many more.
In almost all cases the defrost cycle time is fixed usually 30 minutes.
So defrost starts. The defrost heater comes on. When the defrost thermostat (clipped to the top of the evaporator coils) gets above freezing (all the ice gone) it opens and kills power to the heater. The defrost cycle continues for the rest of the 30 minutes but the heater is off.
So you may have a problem with the defrost thermostat.
Some units have baffles/stops on the side of the evaporator coil that directs the heat up onto the coils which can cause problems if they move or disintegrate.
May also be that you are opening the door often, a bad door seal (either the fresh food or freezer) or there is high humidity. This will form more frost/ice on the coils so it will need the heater to be on for a longer time to get rid of it. This adds more heat to the unit and that will be seen in the fresh food section once the fans come on.
Or it may just be the nature of the beast.