I recharge mine when it's deep brown. It used to take me a couple of days of soaking in bleach, rinsing, soaking in bleach again.... And then another day of soaking in a strong dechlorinator solution, rinsing, soaking again...
Then a guy at one of my lfs suggested using pool chlorine. I bought the powder, and not realizing the strength of it, melted the plastic container I put the solution in. Lol
I still don't know how strong I ought to be making the solution, but the purigen is recharged in an hour. I rinse rinse rinse and then soak in a strong solution of Safe (like prime) for a few hours. I give it a thorough rinse again, and its good to go.
Initially I used to allow it to dry out so I could smell it for any residual chlorine.