There's no "setting " of the defrost timer, it keeps its own time.
Your unit is comprised of a 3 component defrost system
including the timer, bi-metal, and a heater.
If you have excessive frost build up on the evaporator
coils or it's clogged completely then yes you have a defrost
now all you have to do it figure out which of the components you need.
the defrost timer is located in the control housing inside of the
refrigerator section and yes there is a hole there to advance the timer for testing.
Item # 646 on diagram below
if your coils are still clogged all you have to do is slowly advance the timer
clockwise with a screw driver "click by click" and when you reach the defrost mode you will
hear the compressor and freezer fan shut off.
When in defrost mode if the heater thats around the evaporator coils
imediately comes on the you have a bad timer. the heater can be checked by
touch "be careful" or with an amp meter.
If the heater is not getting hot then you either have a bad bi-metal
or a bad heater. the heater can simply be OHM'd out for continuity
and so can the bi-metal (as long as it's still covered with ice/frost)
If it's not you can submerge it into a glass of ice water for a few minutes.
Timer Link (if needed) Bi-Metal Link (if needed) Heater Link (if needed)
Source: How To Set Hotpoint CTX24GLR Defrost Control