[Randy Holmes-Farley]

I presume that the concern with leaving carbon in a long time is that it may develop a coating of bacteria that may facilitate the conversion of ammonia to nitrate. That might be true, but in my tank nitrate is never a concern as I export plenty of nitrogen via growing macroalgae.

But if you want to avoid that issue, changing it faster can work. I generally recommend 1-2 months on carbon used to reduce organics and yellowing of the water, however.