I've used Purigen in several tanks for approx. 5 years, and regenerate when the Purigen beads become medium-brownish, usually around 2-3 weeks.
IMO Purigen will have lost all, or almost all, of its adsorption capacity quite a while before it reaches a black color.
This may prevent it from working to best efficiency even after being regenerated, since the bleaching process overnite cannot deal properly with removing most of the impurities which have been accumulated. To me, proof is in the color of the bag on the right which has been re-charged. It's coloration is quite dark - it should really be nearly white, almost as white as when new/unused.
That's my view.

 

I am going to agree with Discus, my Purigen has NEVER gotten black. It does get about the color of the bag on the right and that is when I regen it.

 

Just one clarification, the pic looks black, in person it is more of a coffee grounds dark brown

 

I feel it's still too dark to work effectively.
Try regenerating when it gets no darker than medium brownish - I think you'll find that it regenerates to a very light beige color so you know that almost all impurities are removed.
I often re-charge with full strength bleach for over 24 hours - then rinse thoroughly several times with conditioned water, and don't let it dry out before re-using - make sure it's kept damp.
You can place it in a zip lock bag to keep it moist, or let it sit in a bucket covered with fresh conditioned water.

 

The general consensus from the reefing community is use it once till its done and Chuck it. Ive found I've had much better luck with this after a few years of regeneration. It seems to work a lot better when new. Who knows though. I might just let it too far gone the first time.

 

Most people in the Reef community also don't want to add any possibility of leeching bleach into the tank as well, which I would be cautious about with that big of an investment. I have heard a few people say that the have trouble after some time on here but I have never had that issue, I still have a 5 year old bag that seems to work fine after bleaching. I do use 100% bleach over night, however.

That said, I tend to not use this product anymore, now that I have all canister filters. I used to clean it monthly, if not more often but I only clean my filters every 6 months or so which means I likely only get the benefit for about a month, if that, twice a year.

 

Too risky for me to regenerate and risk cleaning all my stock. I always just spend the few bucks to get a new bag. But that's quite impressive how dark that got !!

 

It was once and done for me also. I didn't want the chance of getting bleach into the tank and I replaced it long before it was as dark as what we're seeing in the pic. I've not used Purigen in a few months though and I can't see any difference with the tank. Water is just as clear as before and the fish and plants seem unchanged as well.

You really don't need to be concerned over the safety of your fish by getting bleach in the tank - you simply need to rinse the Purigen bag well, several times in conditioned water after re-charging before the next use. The bleach dissipates completely, very quickly, with fresh water rinsing.
And you'd save a lot of $$$ too.

As I think I've mentioned, I've used Purigen 24/7 in several tanks for about 5 years and never had a problem.
In my 75 gal discus tank, I use 2 bags continuously in 2 separate HOB filters, rotating the re-charging of each bag.
That tank has held as many as 12 juvie discus at a time, and I've not lost any, except one by jumping. And you folks know how costly discus are, so one can't afford to lose any by taking undue risks.
And I re-charge each bag every 2 or 3 weeks - for up to a dozen times or more before they begin to lose quite a bit of effectiveness & get replaced with new. A 250 ml bottle of Purigen will last me for many months that way, making the use of Purigen very inexpensive.
Give it a try.

 

It was once and done for me also. I didn't want the chance of getting bleach into the tank and I replaced it long before it was as dark as what we're seeing in the pic. I've not used Purigen in a few months though and I can't see any difference with the tank. Water is just as clear as before and the fish and plants seem unchanged as well.