I've spent a LOT of time on water so I'll add some and take a few issues with some other comments.
The chlorine in Clorox declines a percent or so per month, depending on storage temp. Keep it cool and it doesn't degrade as fast. However, even after a year or more it still has enough Chlorine to purify water - it just takes more. If your emergency water source has a lot of organic compounds in it you will need more chlorine. The chlorine reacts with the organic compounds (and pathogens) and is used up. What you are after is "residual chlorine". When you have a little of that (a few ppm) your water is safe. That's why most municipal tap water has a chlorine taste and smell.
So how much chlorine to use when the chlorine concentration of bleach is declining with age? And then you also have to know how much organic compounds (these often contain the pathogens) are in the water?
As previously mentioned, there are simple and low cost residual chlorine test strips you can get. Also see:
The strips are much cheaper than a chlorine test meter
Regardless, in an emergency situation MORE chlorine is far better than not enough as this is not the time to be getting a water-borne illness. You can simply tell if you have enough chlorine by adding it, stir it in, let it sit for an hour and then smelling it. If it smells like chlorine you have residual chlorine. With relatively new (unscented) Clorox and relatively clean water, start with 8 drops/gallon and move up from there. Keep adding a few more drops at a time, repeating the mixing and waiting, until the water has a chlorine smell.
BTW, chlorine reacts with organic compounds to create a cancer causing class of chemical compounds. HOWEVER, you can easily filter them, and the chlorine, out by passing the water through a solid block carbon filter.
Cryptosporidium and giardia cysts are VERY HARD to kill and both are in most raw sources of ground water, and even in many wells. While its a time versus concentration thing, I have a report (somewhere) that crypto cysts will survive for an hour in pure Clorox. Regular chlorine treatment for 24 hours will NOT kill cryptosporidium.
While neither will kill MOST healthy people, you don't want to be getting either in a SHTF or TEOTWAWKI situation. The only practical prepper way to get rid of the buggers is to mechanically filter them out like your municipal water company usually does. (Strong UV light also kills them.) That takes a filter with a maximum ABSOLUTE 10 micron filter size. Fortunately, those are cheap. A carbon block filter has a much smaller pore size and it does not cost that much more. However, too much "crud" will clog a carbon block filter so you want to put a 1-10 micron filter in front of your carbon block filter. If your emergency water is really dirty, put a 25-100 micron in front of those. Or pre-filter your water through coffer maker filters.
Best place I've found for DIY filters is www.premium-water-filters.com
Be sure to see their low cost emergency gravity filter:
BTW2, other than a water wasting and energy intensive membrane filter, mechanical filters will NOT filter out a virus. So if H5N1 infected birds are paddling around in your water source, add chlorine!
BTW3, Premium Water Filters has a new class of filter that was developed for the military that, allegedly, will filter out a virus.
As noted earlier, better than Clorox, get some calcium hypochlorite. Ca(CLO)2. That is the primary ingredient in MOST Pool Shocks. A pound will treat 10,000 gallons. Make sure the Pool Shock contains nothing but calcium hypochorite.
CAUTION: calcium hypochorite is not Play Dough. In the absolute wrong conditions it can go BOOM. CAREFULLY follow the handling instructions and be sure to use eye protection and a breathing filter/mask. Don't put the stuff in a metal container, including a glass jar with a metal lid, and store it OUTSIDE.
If you can find any of those old plastic film canisters, divide the pound up into some and then put them all in another plastic container.
I haven't researched sodium dichloro-s-triazinetrione, so I can't comment on it but I'm in the "how long has the stuff been around and been used" camp rather than the "oops - call 1-800-sue the bastards" camp.
For more info on water treatment, see: http://www.globalwaterintel.com/archive/8/10/market-insight/chlorines-glory-days.html