How long before replacing Carbon filter ?

[batang_mcdo]

tap water that goes into my tank during water change goes through a carbon filter.
how do i know if its time to change the carbon ? even with the carbon i usually also dose with anti chlorine.
but planning to start my drip system. how do i determine if it needs changing already?

 

[Mr Pleco]

I use chlorine and amm test kits on output of my drip system .. .... drip system best investment I ever made on my tanks..