Under gravel carbon change Frequency?

Just wondering how often I should change the carbon filter in the vertical head post of the under gravel filter system in my 55 gal tank? My unit has has 4. The unit came with a sealed carbon heads which I have managed to separate with some care full use of an xacto knife for the purpose of providing replacement carbon.
Should I treat this as a hang on tank filter schedule of every 30 days, more, or less frequently for change out purposes? Obviously there is less carbon in these small heads and they are also not receiving the flow rate that which most of the hang on filter cartridge receives during the same time period.
Does the carbon wear out or become contaminated beyond use after a period of time?
Appreciate any thoughts on the subject. Goal is to provide the best filtration possible within a reasonable cost. Also reading that any filter washing should be done with cold water as hot water may destroy the good bacteria in the filter? True or False?