I've taken the carbon out of my filter as I'm treating the tank with malachite green. Can I put this carbon back in the filter once I'm done, or should replace it?

I'm just wondering how long it lasts? It's been in there since I started the tank about 6 weeks ago?

I only use carbon to remove medications. Carbon potency only lasts for a few days so chances are the carbon you removed is no longer useful if it was being used for 6 weeks.

I use my carbon for like a month and my water is clear it seems to be fine

Once the "usefulness" of the carbon is spent there is no need to remove it if you do not wish too. It simply serves as a nice, porous platform for your good bacteria to colonize.