Carbon should just be used when you're trying to remove medications after treatment. It's only active for a few days, and otherwise can actually interfere with the biological colony's growth. Just take out the carbon and use the floss, or some other media.
I treat the carbon in the juwel filter like any other sponge a home for the bactria, if i get carbon in bags for externals i remove it and fill the try with noodles
How long carbon lasts depends on many things including the quality of it, the quantity of it, and the amount of chemicals in the tank it will be removing.
Carbon can be recharged in general, but not with any methods most people have access to.