I personally don't have carbon in my filter at all.
As you are aware, carbon needs changing every few weeks, else it loses its efficacy. Its primary use is to remove heavy metal pollutants. However, a decent dechlorinator (and a lot of rubbish ones
) will remove low concentrations of those pollutants anyway, so under normal usage, there is nothing for the carbon to do.
Where carbon is a good idea is when you have dosed the aquarium with medication (many fish meds are based on heavy metals such as copper), and so I keep carbon in the cupboard ready for such a time.
I would say the only circumstances where you would need to habitually use carbon is where you do not use a mains-fed water supply, but instead use a well (or similarly untreated) water which may have relatively high levels of such pollutants.