I tend to only use it when removing medicine from the water after a treatment, in which case I'll usually toss it after using it a couple uses (for two days or so each use). Not sure if that's the best way, but it seems to work.
I am thinking about just running it only after a WC for a couple hours, to remove all the crap the water company put into the tab water. But I would like to save the carbon and reuse it next week again when another WC. It seems kinda wasteful to throw it away just after a couple hours of usage.
Yeah a month is about all carbon can take, but you really shouldn't need to bother with it as long as you're adding proper water conditioner during your waterchanges.