To tell when carbon filters are exhausted, the best way is to use an indoor VOC monitor.
Air filter AC is specially prepared, so making your own DIY charcoal (while possible) isn't nearly as effective. You can mostly regenerate/desorb the carbon by heating it to 400F. Never use your regular oven though, not least of which because it reeks. Get a cheap toaster oven and disposable foil pans and do it outside, in a location where it can't possibly catch anything on fire.
