Planted Tank Guru
Join Date: Jul 2010
Location: British Columbia, Canada
Yes, but don't leave it till it gets very dark brown/blackish. It will then have already begun to lose it's effectiveness to adsorb undesirable material if you do, it will take longer to fully re-generate properly, and I expect it would have a shorter useful lifespan overall.
Re-charge after it gets light/medium brown. Depending on the size of tank and the water conditions & quality, that could be anywhere from approx. 3 to 6 weeks as an average.
I re-charge mine after 2-3 weeks.